Network Working Group                                     H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: August 15, 2004                               February 15, 2004

                   The tel URI for Telephone Numbers

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Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document specifies the URI (Uniform Resource Identifier) scheme
   "tel".  The ``tel'' URI describes resources identified by telephone

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.    URI Syntax . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.    URI Comparisons  . . . . . . . . . . . . . . . . . . . . . .  6
   5.    Phone Numbers and Their Context  . . . . . . . . . . . . . .  7
   5.1   Phone Numbers  . . . . . . . . . . . . . . . . . . . . . . .  7
   5.1.1 Separators in Phone Numbers  . . . . . . . . . . . . . . . .  7
   5.1.2 Alphabetic Characters Corresponding to Digits  . . . . . . .  8
   5.1.3 Alphabetic, * and \\# Characters as Identifiers  . . . . . .  8
   5.1.4 Global Numbers . . . . . . . . . . . . . . . . . . . . . . .  8
   5.1.5 Local Numbers  . . . . . . . . . . . . . . . . . . . . . . .  8
   5.2   ISDN Subaddresses  . . . . . . . . . . . . . . . . . . . . . 10
   5.3   Extensions . . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.4   Other Parameters . . . . . . . . . . . . . . . . . . . . . . 11
   6.    Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.    Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.1   Why Not Just Put Telephone Numbers in SIP URIs?  . . . . . . 11
   7.2   Why Not Distinguish Between Call Types?  . . . . . . . . . . 12
   7.3   Why tel? . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   7.4   Do Not Confuse Numbers with How They Are Dialed  . . . . . . 12
   8.    Usage of Telephone URIs in HTML  . . . . . . . . . . . . . . 12
   9.    Use of tel URIs with SIP (Informative) . . . . . . . . . . . 13
   9.1   Local Translation  . . . . . . . . . . . . . . . . . . . . . 14
   9.2   Proxy Translation  . . . . . . . . . . . . . . . . . . . . . 15
   10.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 15
   11.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 16
   12.   Security Considerations  . . . . . . . . . . . . . . . . . . 16
   13.   Changes Since RFC 2806 . . . . . . . . . . . . . . . . . . . 16
         Normative References . . . . . . . . . . . . . . . . . . . . 16
         Informative References . . . . . . . . . . . . . . . . . . . 17
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 18
         Intellectual Property and Copyright Statements . . . . . . . 19

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1. Introduction

   This document defines the URI scheme "tel".  The "tel" URI describes
   resources identified by telephone numbers.  A telephone number is a
   string of decimal digits that uniquely indicates the network
   termination point.  The number contains the information necessary to
   route the call to this termination point.  (This definition is
   derived from [E.164], but encompasses both public and private

   The "tel" URI telephone number is not restricted in the type of
   termination point it refers to.  The termination point can be in the
   public telephone network, a private telephone network or the
   Internet. The termination point can be fixed or wireless and address
   a fixed wired, mobile or nomadic terminal.  The terminal addressed
   can support any electronic communication service (ECS) including
   voice, data and fax.  The URI can refer to resources identified by a
   telephone number, including but not limited to originators or targets
   of a telephone call.

   The "tel" URI is a globally unique identifier ("name") only; it does
   not describe the steps necessary to reach a particular number and
   does not imply dialing semantics. Furthermore, it does not refer to a
   specific physical device, only to a telephone number.

   Telephone numbers as commonly understood actually comprise two
   related, but distinct concepts: as a canonical address-of-record and
   as a dial-string. We define the concepts below:

   Address-of-record or identifier: The telephone number is understood
      here as the canonical address-of-record or identifier for a
      termination point within a specific network.  For the public
      network, these numbers follow the rules in E.164 [E.164], while
      private numbers follow the rules of the owner of the private
      numbering plan.  Subscribers publish such identifiers phone number
      as a universal means of being reached, independent of the location
      of the caller.  (Naturally, not all numbers are reachable from
      everywhere, for a variety of technical and local policy reasons.
      Also, a single termination point may be reachable from different
      networks and may have multiple such identifiers.)
   Dial string: "Dial strings" are the actual numbers, symbols and
      pauses entered by a user to place a phone call.  A dial-string is
      consumed by one or more network entities, and understood in the
      context of the configuration of these entities.  It is used to
      generate a telephone number so that a call can be routed.
      Dial-strings may require pre-pended digits to handle local PBXs,
      and they may include post-dial DTMF signaling that could control
      an IVR or reach an extension.  Dial strings are beyond the scope

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      of this document.

   To reach a telephone number from a phone on a PBX, for example, the
   user of that phone has to know how to convert the telephone number
   identifier into a dial string appropriate for that phone.  The
   telephone number itself does not convey what needs to be done for a
   particular terminal. Instructions may include dialing "9" before
   placing a call or prepending a "00" to reach a number in a foreign
   country.  The phone may also need to strip area and country codes.

   The notation for phone numbers in this document is similar to that in
   RFC 3191 [RFC3191] and RFC 3192 [RFC3192].  However, the syntax
   differs since this document describes URIs whereas RFC 3191 and RFC
   3192 specify electronic mail addresses. RFC 3191 and RFC 3192 use "/"
   to indicate parameters (qualifiers). Since URI use the forward slash
   to describe path hierarchy, the URI scheme described here uses the
   semicolon, in keeping with Session Initiation Protocol (SIP) URI
   conventions [RFC3261].

   There are at least two ways one can envision making a telephone
   connection.  In the first approach, a URI contains the dial string,
   which is then passed to an entity that can reproduce the actions
   specified in the dial string.  For example, in an analog phone
   system, a dialer translates the dial string into a sequence of
   actions such as waiting for dial tone, sending DTMF digits, pausing
   and generating post-dial DTMF digits after the callee picks up.  In
   an ISDN or ISUP environment, the recipient of the dial string
   performs the appropriate protocol actions.

   Another approach has the URI specify the telephone number, which can
   be either globally unique or only be valid within a local context.
   The dialing application is aware of the local context, knowing, for
   example, whether special digits need to be dialed to seize an outside
   line, whether network, pulse or tone dialing is needed and what tones
   indicate call progress.  The dialing application then converts the
   telephone number into a dial sequence and performs the necessary
   signaling actions.  The document below assumes the second model.  The
   dialer does not have to be a user application as found in traditional
   desktop operating systems, but could well be part of an IP-to-PSTN

   The approach pursued here has the disadvantage that certain services,
   such as electronic banking or voicemail, cannot be specified in a

   The URI can be used as a request URI in SIP [RFC3261] requests.  The
   SIP specification also inherits the 'subscriber' part of the syntax
   as part of the 'user element' in the SIP URI.  Other protocols may

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   use this URI for both query-based and prefix-based applications.

   The "tel" URI does not specify the call type such as voice, fax, or
   data call and does not provide the connection parameters for a data
   call.  The type and parameters are assumed to be negotiated either
   in-band by the telephone device or through a signaling protocol such
   as SIP.

2. Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant

3. URI Syntax

   The URI is defined using the ABNF (augmented Backus-Naur form)
   described in RFC 2234 [RFC2234] and uses elements from the core
   definitions (Appendix A of RFC 2234).

   The syntax definition follows RFC 2396 [RFC2396], indicating the
   actual characters contained in the URI.  Note that the reserved
   characters "+", ";", "=", and "?" MUST NOT be escaped if shown in the
   grammar definitions below as they are delimiters for the "tel" URI
   scheme.  These reserved characters MUST be escaped if they appear in
   parameter values.

   Characters other than those in the "reserved" and "unsafe" sets (see
   RFC 2396 [RFC2396]) are equivalent to their "% HEX HEX" encoding.

   The "tel" URI has the following syntax:

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   telephone-uri        = "tel:" telephone-subscriber
   telephone-subscriber = global-number / local-number
   global-number        = global-number-digits *par
   local-number         = local-number-digits *par context *par
   par                  = parameter / extension / isdn-subaddress
   isdn-subaddress      = ";isub=" 1*uric
   extension            = ";ext=" 1*phonedigit
   context              = ;phone-context=" descriptor
   descriptor           = domainname / global-number-digits
   global-number-digits = "+" 1*phonedigit
   local-number-digits  = 1*phonedigit-hex
   domainname           = *( domainlabel "." ) toplabel [ "." ]
   domainlabel          = alphanum
                          / alphanum *( alphanum / "-" ) alphanum
   toplabel             = ALPHA / ALPHA *( alphanum / "-" ) alphanum
   parameter            = ";" pname ["=" pvalue ]
   pname                = 1*( alphanum / "-" )
   pvalue               = 1*paramchar
   paramchar            = param-unreserved / unreserved / escaped
   unreserved           = alphanum / mark
   mark                 = "-" | "_" | "." | "!" | "~" | "*" |
                          "'" | "(" | ")"
   escaped              = "%" HEXDIG HEXDIG
   param-unreserved     = "[" / "]" / "/" / ":" / "&" / "+" / "$"
   phonedigit           = DIGIT / [ visual-separator ]
   phonedigit-hex       = HEXDIG / "*" / "#" / [ visual-separator ]
   visual-separator     = "-" / "." / "(" / ")"
   alphanum             = ALPHA / DIGIT

   Each parameter name ("pname"), the ISDN subaddress, the extension and
   the context MUST NOT appear more than once.  The order of the URL
   parameters is immaterial.  The ISDN subaddress or extension SHOULD
   appear first, if present, followed by the context parameter, if
   present, followed by any other parameters in lexicographical order.

      This simplifies comparison when the "tel" URI is compared
      character-by-character, such as in SIP URIs [RFC3261].

4. URI Comparisons

   Two "tel" URIs are equivalent according to the following rules:

   o  URI are not equal if one is a 'local-number and the other a
   o  For mandatory additional parameters (Section 5.4) and the
      'phone-context' and 'extension' parameters defined in this
      document, 'phone-number' parameter values are compared
      digit-by-digit after removing all 'visual-separator's from

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   o  Parameters are compared according to 'pname', regardless of the
      order they appeared in the URI. If one URI has a parameter name
      not found in the other, the two URIs are not equal.
   o  URI comparisons are case-insensitive.

   All parameter names and values SHOULD use lower-case characters since
   tel URIs may be used within contexts where comparisons are

      Section 19.1.4 in the SIP specification [RFC3261] discusses one
      such case.

5. Phone Numbers and Their Context

5.1 Phone Numbers

   The 'subscriber part of the URI indicates the number.  The phone
   number can be represented in either global (E.164) or local notation.
   All phone numbers MUST use the global form unless they cannot be
   represented as such.  Numbers from private numbering plans, emergency
   ("911", "112") and some directory assistance numbers (e.g., "411")
   and other "service codes" (numbers of the form N11 in the United
   States) cannot be represented in global (E.164) form, and need to be
   represented as a local number with a context.  Local numbers MUST be
   tagged with a 'phone-context' (Section 5.1.5).

   Implementations MUST NOT assume that telephone numbers have a
   maximum, minimum or fixed length, or that they always begin with a
   certain number.

      E.164 limits numbers to 15 digits.  For geographic numbers, one to
      three digits are the country code, with the remainder divided into
      area or city code (national destination code) and subscriber
      number.  Alternatively, there is a global three-digit service
      code, followed by a global subscriber number of up to 12 digits.
      Finally, a "international public telecommunication number for
      networks is composed of decimal digits arranged in three code
      fields.  The code fields are the 3-digit shared Country Code (CC)
      field, the IC field, which varies in length between 1 to 4 digits,
      and the Subscriber Number (SN) which can be up to 15 minus the
      number of digits in the CC and IC fields." [E.164].

5.1.1 Separators in Phone Numbers

   Phone numbers MAY contain visual separators.  Visual separators
   ('visual-separator') merely aid readability and are not used for URI
   comparison or placing a call.

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      Despite complicating comparisons, this specification retains the
      visual separators to follow the spirit of RFC 2396 [RFC2396],
      which remarks that "A URI often needs to be remembered by people,
      and it is easier for people to remember a URI when it consists of
      meaningful components." Also, ISBN URNs documented in RFC 3187
      [RFC3187] use visual separators in a manner similar to this

      Even though ITU-T E.123 [E.123] recommends the use of space
      characters as visual separators in printed telephone numbers,
      "tel" URIs cannot use spaces to avoid excessive escaping.

5.1.2 Alphabetic Characters Corresponding to Digits

   In some countries, it is popular to write phone numbers using
   alphabetic characters which correspond to certain numbers on the
   telephone keypad.  The URI format does not support this notation
   since the mapping from alphabetic characters to digits is not
   completely uniform internationally, although there are standards
   [E.161][T1.703] addressing this issue.

5.1.3 Alphabetic, * and \\# Characters as Identifiers

   Since called and calling terminal numbers (TNs) are encoded in BCD in
   ISUP, six additional values per digit can be encoded, sometimes
   represented as the hexadecimal characters A through F.  Similarly,
   DTMF allows for the encoding of the symbols *, \# and A through D.
   However, in accordance with E.164, they may not be included in global
   numbers. Their use in local numbers is not defined, but is not

5.1.4 Global Numbers

   Globally unique numbers are identified by the leading "+" character.
   Global numbers MUST be composed with the country (CC) and national
   (NSN) numbers as specified in E.123 [E.123] and E.164 [E.164].
   Globally unique numbers have the property of being unambiguous
   everywhere in the world and are RECOMMENDED.

5.1.5 Local Numbers

   Local numbers are unique only within a certain geographical area or a
   certain part of the telephone network, e.g., a private branch
   exchange (PBX), a state or province, a particular local exchange
   carrier or a particular country.  URIs with local phone numbers
   should only appear in environments where all local entities can
   successfully set up the call by passing the number to the dialing
   software.  Digits needed for accessing an outside line, for example,

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   are not included in local numbers.  Local numbers SHOULD NOT be used
   unless there is no way to represent the number as a global number.

      Local numbers require that the originator and recipient are
      configured appropriately, so that they can insert and recognize
      the correct descriptors.  Since there is no algorithm to
      independently pick the same descriptor, labeling numbers with
      their context increases the chances of mis-configuration, so that
      valid identifiers are rejected by mistake.  The algorithm to
      select descriptors was chosen that accidental collisions should be
      rare, but they cannot be ruled out.

   Local numbers MUST have a 'phone-context' parameter that identifies
   the scope of their validity.  The parameter MUST be chosen to
   unambiguously identify the local context within which the number is
   unique.  Thus, the combination of the descriptor in the
   'phone-context' parameter and local number is again globally unique.
   The parameter value is defined by the assignee of the local number.
   It does NOT indicate a prefix that turns the local number into a
   global (E.164) number.

   There are two ways to label the context:  via a global number or any
   number of its leading digits (e.g., "+33") and via a domain name,
   e.g., "".  The choice between the two is left to
   the "owner" of the local number and is governed by whether there is a
   global number or domain name that is a valid identifier for a
   particular local number.

   The domain name does not have to resolve to any actual host, but MUST
   be under the administrative control of the entity managing the local
   phone context.

   A global number context consists of the initial digits of a valid
   global number.  All global numbers matching these initial digits must
   be assigned to the same organization that is describing the context
   and no such matching number can be used by any other organization.
   If such an initial string of digits does not exist, the organization
   should use the lowest number of the global number range assigned to
   it.  (This can occur if two organizations share the same decimal
   block of numbers.  For example, assume an organization owns the
   number range +1-212-939-7000 through +1-212-939-7199.  +1-212-939-7
   would not be a valid global number context, but +1-212-939-7000 would
   work.) It is not required that local numbers within the context
   actually begin with the chosen set of initial numbers.

   For a local number defined within a PBX, the organization can choose
   any number under its control to identify the context.  For example, a
   context consisting of any of the organization's global numbers may be

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   suitable, or a substring that is completely occupied by the
   organization.  For example, +49-6151-16 would be a suitable context
   for the TU Darmstadt, as it uses all numbers starting with those

   A context consisting of the initial digits of a global number does
   not imply that adding these to the local number will generate a valid
   E.164 number.  It might do so by coincidence, but this cannot be
   relied upon. (For example, "911" should be labeled with the context
   "+1", but "+1-911" is not a valid E.164 number.)

   National freephone numbers do not need a context, even though they
   are not necessarily reachable from outside a particular country code
   or numbering plan.  Recall that "tel" URIs are identifiers; it is
   sufficient that a global number is unique, but it is not required
   that it be reachable from everywhere.

      Even non-freephone numbers may be out of date or not be reachable
      from a particular location.  For example, premium services such as
      "900" numbers in the North American numbering plan are often not
      dialable from outside the particular country code.

      The two label types were chosen so that, in almost all cases, a
      local administrator can pick an identifier that is reasonably
      descriptive and does not require a new IANA-managed assigned
      number.  It is up to the administrator to assign an appropriate
      identifier and to use it consistently.  Often, an organization can
      choose among several different identifiers.

   If the recipient of a "tel" URI uses the URI simply for
   identification, the receiver does not need to know anything about the
   context descriptor.  It simply treats it as one part of a globally
   unique identifier, with the other being the local number.  If a
   recipient of the URI intends to place a call to the local number, it
   MUST verify that it is within the same context as one of the
   descriptors.  If it is not within the same context, it MUST NOT
   initiate the call and treat the URI like an invalid destination.

5.2 ISDN Subaddresses

   A phone number MAY also contain an 'isdn-subaddress"> parameter which
   indicates an ISDN subaddress.

   ISDN subaddresses typically contain IA5 characters, but may contain
   any octet value.

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5.3 Extensions

   Extensions identify stations behind a PBX and are roughly equivalent
   to ISDN subaddresses.  They are identified with the 'extension">
   parameter.  At most one of the 'isdn-subaddress and 'extension
   parameters can appear in a tel URI, i.e., they cannot appear both at
   the same time.

5.4 Other Parameters

   Future extensions to this URI scheme may add other parameters
   ('parameter in the ABNF).  Such parameters can be either mandatory or
   optional.  Mandatory parameters start with "m-".  An implementation
   MAY ignore optional parameters.  An implementation MUST NOT use the
   URI if it contains unknown mandatory parameters.  The "m-" prefix
   cannot be added to parameters that were already registered (except to
   create a new, logically distinct parameter).  The "phone-context"
   parameter in this document is mandatory.

   For example, 'parameter' parameters can be used to store
   application-specific additional data about the phone number, its
   intended use, or any conversions that have been applied to the

   Entities that forward protocol requests containing tel URIs with
   optional parameters MUST NOT delete or modify parameters they do not

   All new parameters MUST be registered with IANA.

6. Examples

   tel:+1-201-555-0123 This URI points to a phone number in the United
      States.  The hyphens are included to make the number more
      human-readable; they separate country, area codes and subscriber
   tel:7042; The URI describes a local
      phone number valid within the context "".
   tel:863-1234;phone-context=+1-914-555: The URI describes a local
      phone number that is valid within a particular phone prefix.

7. Rationale

7.1 Why Not Just Put Telephone Numbers in SIP URIs?

   The "tel" URI describes a service, reaching a telephone number, that
   is independent of the means of doing so, be it via a SIP-to-PSTN
   gateway, a direct SIP call via ENUM translation, some other signaling

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   protocols such as H.323 or a traditional circuit-switched call
   initiated on the client side via, say, TAPI.  It is thus, in spirit,
   closer to the URN schemes that also leave the resolution to an
   external mechanism. The same "tel" URI may get translated to any
   number of other URIs in the process of setting up the call.

7.2 Why Not Distinguish Between Call Types?

   Signaling protocols such as SIP allow to negotiate the call type and
   parameters, making the very basic indication within the URL scheme
   moot. Also, since the call type can change frequently, any such
   indication in a URI is likely to be out of date. If such designation
   is desired for a device that directly places calls without a
   signaling protocol such as SIP, mechanisms such as the "type"
   attribute for the "A" element in HTML may be more appropriate.

7.3 Why tel?

   "Tel" was chosen since it is widely recognized none of the other
   suggestions appeared appropriate.  "Callto" was discarded since URI
   schemes locate a resource and do not specify an action to be taken.
   "Telephone" and "phone" were considered too long and not as
   internationally recognized.

7.4 Do Not Confuse Numbers with How They Are Dialed

   As an example, the E.164 number "+1-212-555-3141" will be dialed in
   many countries as 00-1-212-555-3141, where the leading "00" is a
   prefix for international calls.  (In general, "+" in E.164 indicates
   that an international prefix is required.) Tel URIs MUST NOT contain
   the local dialing prefixes in numbers such as +1-212-555-3141, as the
   transformation back to an international number is not guaranteed to
   be correct or unique.

   If a network entity receives a "tel" URI containing a local number,
   it MUST make sure that it knows the context in which the local phone
   number is to be processed, or else the number MUST NOT be used.
   Equally, the originator of a "tel" URI must take into consideration
   that the recipient may have insufficient information about the phone
   number's context.

8. Usage of Telephone URIs in HTML

   Links using the "tel" URL SHOULD enclose the telephone number, so
   that users can easily predict the action taken when following the

   Dial <a href="tel:+1-212-555-0101">+1-212-555-0101</a>

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   for assistance.

   instead of

   Dial <a href="tel:+1-212-555-0101">this number</a>
   for assistance.

   On a public HTML page, the telephone number in the URI SHOULD always
   be in the global form, even if the text of the link uses some local

   Telephone (if dialing in the United States):
     <a href="tel:+1-201-555-0111">(201) 555-0111</a>

   or even

   For having RFCs read aloud, call
   <a href="tel:+1-555-438-3732">1-555-IETF-RFC</a>.

9. Use of tel URIs with SIP (Informative)

   SIP can use the "tel" URI as a Request-URI, along with "sip" and
   "sips" URIs.  For brevity, we will imply "sips" URIs when talking
   about SIP URIs.  Both "tel" and SIP URIs can contain telephone
   numbers.  In SIP URIs, they appear as the user part, i.e., before the
   @ symbol (Section 19.1.6 in [RFC3261]).

   Unless a SIP UA connects directly to a PSTN gateway, one of the SIP
   proxy servers has to translate the tel URI to a SIP URI, with the
   host part of that URI pointing to a gateway.  Typically, the outbound
   proxy server, as the first proxy server visited by a call request,
   performs this translation.  A proxy server can translate all tel URIs
   to the same SIP host name or select a different gateway for different
   tel prefixes, based, for example, on information learned from TRIP.
   However, a proxy server could also delegate this translation task to
   any other proxy server since proxy servers are free to apply whatever
   routing logic they desire.

   As noted earlier, all phone numbers MUST use the global form unless
   they cannot be represented as such.  If the local-number format is
   used, it MUST be qualified by the 'phone-context' parameter.
   Effectively, the combination of local number and phone context makes
   the tel URI globally unique.

   While web pages, vCard business cards, address books and directories
   can easily contain global tel URIs, users on twelve-button (IP)
   phones cannot dial such numbers directly and are typically accustomed

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   to dialing shorter strings, e.g., for PBX extensions or local
   numbers. These so-called dial-strings (Section 1) are not directly
   represented by tel URIs, as noted.  We refer to the translation of
   dial strings into SIP and tel URIs, global or local, as the dial
   plan.  There are at least two appropriate ways to deal with dial
   strings in SIP terminals, local translation and proxy translation,
   described in turn below.

9.1 Local Translation

   A SIP UA can use a dial plan to translate dial strings into SIP or
   "tel" URIs.  The dial plan can be manually configured or, preferably,
   be downloaded as part of a device configuration mechanism.  (At this
   time, there is no standardized mechanism for this.)

   A mobile user can use at least two dial plans, namely the dial plan
   for the network that he is currently visiting and the dial plan for
   the user's home network.  Generally, dialed numbers that are meant to
   represent global numbers will look the same after the translation
   regardless of the dial plan, even if, say, the visited network uses
   '0' for dialing an 'outside' number and the user's home network uses
   '9', as long as the user is aware of the current dial plan.  However,
   local extensions that do not have a direct global equivalent may well
   behave differently.  To avoid any ambiguity, the dial plan MUST
   insert a suitable 'phone-context' string when performing the
   translation.  If the 'phone-context' is a domain name, there are
   three cases:
   1.  The outbound proxy recognizes the domain name in the SIP URI as
       its local context and can route the request to a gateway that
       understands the local number.
   2.  The outbound proxy does not use the same phone context, but can
       route to a proxy that handles this phone context. This routing
       can be done via a lookup table or the domain name of the phone
       context might be set up to reflect the SIP domain name of a
       suitable proxy. For example, a proxy may always route calls with
       tel URIs like


        to the SIP proxy located at  (Proxies that
       receive a tel URI with a context they do not understand are
       obligated to return a 404 (Not Found) status resonse, so that an
       outbound proxy may decide to attempt such a heuristic.)
   3.  The outbound proxy does not recognize the phone context and
       cannot find the appropriate proxy cognizant of that phone
       context. In that case, the translation fails and the outbound
       proxy returns a 404 (Not Found) error response.

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9.2 Proxy Translation

   In proxy translation mode, the SIP UA simply turns the dialed digits
   into the user part of the SIP URI and appends a ';user=phone'
   parameter and provides an appropriate phone context reflecting the
   local dialing plan.  The host name or IP address of the outbound
   proxy is made the host part of the SIP request URI.  The outbound
   proxy can then apply the dial plan indicated by the phone context in
   the URI to translate the SIP URI into a "tel" URI or other SIP URI.
   Translation into a "tel" URI makes sense only if the proxy wants to
   delegate finding the PSTN gateway to another proxy.  For example,
   after the user with a location-specified dial plan located in domain
   "" dials the digits "1234", the device converts
   this into a SIP URI:


   Alternatively, the SIP UA can issue a call with a "tel" URI. For this
   example, it might be:


   Using a SIP URI is more robust and is thus the preferred approach.

      Since a single proxy may receive calls from many different
      locations with different local dial plans, devices that rely on
      the proxy for number translation SHOULD always be configured with
      a context. Otherwise, for example, a provider or enterprise would
      have to provision a separate proxy for each branch office or
      geographic area with its own dial plan.

10. IANA Considerations

   IANA is requested to update the reference to RFC 2806 in the URI
   scheme registry for the 'tel' scheme to this document.

   "Tel" URI parameters ('parameter') MUST be registered with IANA.
   Mandatory parameters must be described in a standards-track RFC,
   while an informational RFC is sufficient for other parameters.

   The registration must indicate:

   Parameter name: The name used for the parameter, according to the BNF
      in Section 3.
   Applicability: A brief description of its applicability.

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   Mandatory? Whether the parameter is mandatory or not; only the names
      of mandatory parameters must start with "m-" as described in
      Section 5.4.;
   Specification: A reference to the specification that defines the
      parameter and its syntax.

11. Acknowledgments

   This document is derived from RFC 2806 [RFC2806], written by Antti
   Vähä-Sipilä.  Flemming Andreasen, Francois Audet, Lawrence Conroy,
   Cullen Jennings, Michael Hammer, Andrew Main, Xavier Marjou, Jon
   Peterson, Mike Pierce, Jonathan Rosenberg and James Yu provided
   extensive comments.

12. Security Considerations

   The security considerations parallel those for the mailto URL

   A recipient of a "tel" URI SHOULD NOT place calls without the consent
   of its owner.  Placing calls automatically without appropriate user
   confirmation may incur a number of risks, such as those described

   o  Calls may incur costs.
   o  The URI may be used to place malicious or annoying calls.
   o  A call will take the user's phone line off-hook, thus preventing
      its use.
   o  A call may reveal the user's, possibly unlisted, phone number to
      the remote host in the caller identification data, and may allow
      the attacker to correlate the user's phone number with other
      information such as the e-mail or IP address.

13. Changes Since RFC 2806

   The specification is syntactically backwards-compatible with the
   "tel" URI defined in RFC 2806 [RFC2806], but has been completely
   rewritten.  This document more clearly distinguishes telephone
   numbers as identifiers of network termination points from dial
   strings and removes the latter from the purview of "tel" URIs.
   Compared to RFC 2806, references to carrier selection, dial context,
   fax and modem URIs, post-dial strings and pause characters have been
   removed. The URI syntax now conforms to RFC 2396 [RFC2396].

   A section on using tel URIs in SIP was added.

Normative References

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   [E.123]    , ITU., "Notation for national and international telephone
              numbers,  e-mail addresses and web addresses",
              Recommendation E.123, February 2001.

   [E.161]    , ITU., "Arrangement of digits, letters and symbols on
              telephones and other devices that can be used for gaining
              access to a telephone network", Recommendation E.161, May

   [E.164]    , ITU., "The international public telecommunication
              numbering plan", Recommendation E.164, May 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

   [RFC3191]  Allocchio, C., "Minimal GSTN address format in Internet
              Mail", RFC 3191, October 2001.

   [RFC3192]  Allocchio, C., "Minimal FAX address format in Internet
              Mail", RFC 3192, October 2001.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M. and E. Schooler,
              "SIP: Session Initiation Protocol", RFC 3261, June 2002.

   [T1.703]   , ANSI., "Allocation of Letters to the Keys of Numeric
              Keypads for Telecommunications", Standard T1.703-1995
              (R1999), 1999.

Informative References

              Yu, J., "New Parameters for the 'tel' URL to Support
              Number Portability and  Freephone Service",
              draft-yu-tel-url-08 (work in progress), November 2003.

   [RFC2368]  Hoffman, P., Masinter, L. and J. Zawinski, "The mailto URL
              scheme", RFC 2368, July 1998.

   [RFC2396]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC2806]  Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806,
              April 2000.

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   [RFC3187]  Hakala, J. and H. Walravens, "Using International Standard
              Book Numbers as Uniform Resource Names", RFC 3187, October

Author's Address

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027

   Phone: +1 212 939 7042

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