ENUM is a protocol for mapping standard telephone numbers to Internet identifiers. It converts a telephone number into a domain name and then uses the global domain name system (DNS) to retrieve records that associate the number with an identifier that works on the Internet. The identifier could be an Internet Protocol (IP) address, a SIP identifier,
  or a web site URL. ENUM was developed by the Network Working Group of the Internet Engineering Task Force (IETF). The protocol relies on the E.164 telephone numbering standard of the International Telecommunication Union as an assumed structure for all telephone numbers.

 Trends in Telephone Service. U.S. Federal Communications Commission, Common Carrier Bureau, Idustry Analysis Division. August 2001.

 Faltstrom, P., “E.164 number and DNS ,” RFC 2916, September 2000. The ENUM protocol makes extensive use of the Naming Authority Pointer Record (NATPR) to identify different services for, or methods of, contacting a user associated with a telephone number. NAPTR is a new type of DNS resource record. See Mealling, M. and R. Daniel, “The Naming Authority Pointer (NAPTR) DNS Resource Record”, RFC 2915, September 2000.

Session Initiation Protocol (SIP) is defined in RFC xxxx

 International Telecommunication Union, ITU-T Recommendation E.164 , ITU-T Recommendation E.164, “The International Public Telecommunications Numbering Plan,” 1997.

ENUM has generated interest because a growing portion of voice or data traffic may originate or terminate on an IP network. For many years to come, however, a significant amount of telecommunication traffic will remain on traditional circuit-switched,  time-division multiplexed networks. These two types of networks use different signaling and routing systems.
  Interconnecting them requires a fast and efficient method of finding out which type of network a called party is on and what address it is using. The ENUM protocol is intended to solve this problem. It uses DNS queries to allow telecom service providers to discover whether a dialed E.164 number is associated with a telephone company switch, an IP-based network, or something else.

ENUM could prove to be a powerful capability for two reasons. First, it may provide a simpler and less expensive method of interconnecting telecommunication carriers. ENUM services can act as a substitute for Signaling System 7 (SS7) capabilities and the trunk group administration and tandem switching hierarchy used by the PSTN to interconnect service providers. It can act as a bridge between traditional interconnection methods and Internet Protocol-based networks, providing carriers with a migration path between a Time-Division Multiplexing (TDM) and SS7-dominated environment and an IPdominated world.

Second, ENUM provides “intelligent network” capabilities that allow both Internet users and telephone network users to benefit from Internet-based applications. The ENUM protocol will allow telephone numbers to be used to identify many different types of end terminals and associated services, such as cable telephone service, cable fax, and teleconferencing over cable access networks in addition to existing broadband services. Just as SS7 made it possible for voice telephone service to become enhanced with features such as caller ID, distinctive ringing, and the like, so ENUM makes it possible for voice and data services to be linked to a wide variety of multimedia Internet applications that can add functionality to voice and data services. Unified messaging and “follow-me” forwarding services are examples of the types of capabilities that might be enabled by ENUM.