IP telephony is a term used to describe technologies that use the IP protocol to exchange voice, fax, and other forms of information, traditionally carried over the Public Switched Telephone Network (PSTN). The call travels in the form of packets, over a Local Area Network (LAN), or the Internet, avoiding PSTN tolls.
The Internet Protocol becomes the transport for almost all data communications. Today, all communication carriers are using an IP infrastructure for a part, or for all of its voice services.