IETF RFC 4238 PDF

Voice messaging is traditionally addressed using telephone number addressing. This document describes two techniques for routing voice messages based on a telephone number. The complete service uses the Voice Profile for Internet Mail (VPIM) Directory service to lookup a VPIM email address with a telephone number and confirm that the address is both valid and associated with the intended recipient. However, this service will take time to become widely deployed in the near term. This document also describes a basic send-and-pray service that routes and delivers messages using only the ENUM telephone number resolution service and the existing DNS mail routing facilities.