NTT NTT NTT Without a tel URI parameter to carry an encoding type of Integrated Services Digital Network (ISDN) subaddress, interworking between ISDN User Part (ISUP) network and a Session Initiation Protocol (SIP) network is impossible in some cases. To solve this problem, this document specifies a new optional tel URI parameter to carry the encoding type of ISDN subaddress. This memo provides information for the Internet community.