E164 numbers

E.164 - International public telecommunication numbering plan

EPP

RFC 4930 - Extensible Provisioning Protocol (EPP)
RFC 3734 - EPP Transport over TCP

ENUM validation

RFC 4725 - ENUM validation architecture
RFC 5105 - ENUM validation token format definition

DNS

RFC 3761 - E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM)
RFC 3403 - Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database
draft-ietf-enum-edns0 - ENUM Requirement for EDNS0 Support