Somos, Inc. This document defines a mechanism for binding a domain identifier to telephone number authority within a STIR certificate. A certificate produced under this mechanism carries, as a co-validated pair, the telephone numbers or service provider codes a subject is authorized for in a TNAuthList extension and a domain the subject controls in a SubjectAltName dNSName entry. The binding is established at issuance by requiring proof of domain control and validation of a TNAuthList authority token within a single certificate issuance, such that the resulting certificate attests that the same entity holds both. The mechanism applies to STIR certificates whose TNAuthList contains telephone number entries, service provider code entries, or both, allowing a domain to be bound to the right-to-use holder for a set of numbers or to the provider identified by a service provider code. This document defines the issuance conformance requirements and the relying party verification rule that together make the binding meaningful. It does not define telephone number or service provider code authorization or domain validation, both of which are specified elsewhere and referenced here.