One of the principle elements of a future voice over Internet (VoIP) world is called the Enumbering, or Enum, initiative. Simplistically, it is a means of mapping phone numbers to Internet addresses, in the same way that domain names are mapped via the domain naming system (DNS).
It will enable networks to know where you can best be contacted, so re-routing calls as necessary via fixed line, mobile, Web etc – or even to translate a phone call into a voice message and attach it to an e-mail for forwarding.