Being able to dial telephone calls the way customers have come to expect is considered crucial for the convergence of classic telephone service (PSTN) and Internet telephony (VoIP), and for the development of new Internet Protocol (IP) multimedia services. The problem of a single universal personal identifier for multiple communication services can be solved with different approaches. One simple approach is the Electronic Number Mapping System (ENUM) – also known as Telephone Number Mapping. ENUM also permits connecting the IP world to the telephone system in a seamless manner. Read the full background at Enum Wikipedia