Electronic Number Mapping (ENUM) is an IETF Standard ( RFC 3761, was RFC 2916). It defines a DNS-based architecture and protocol by which an E.164 number can be expressed as a Fully Qualified Domain Name (FQDN). The result of an ENUM query is a series of DNS NAPTR records which can be used to contact a resource (e.g. URI) associated with a telephone number.
Electronic Number- is the convergence of PSTN and IP networks;
Telephone Electronic Number Mapping is a system where anyone that has a PSTN phone or a DID can register their phone number on an international database of VoIP users. Whenever someone with a VoIP telephone service (from a participating VSP) calls a phone number using their VoIP service, their participating VSP first checks the ENUM database. Information about ENUM is available on this site on the e164.org page.