IP telephone system
An IP telephone system uses packet-switched Voice over IP (VoIP), or Internet telephony, to transmit telephone calls over the Internet as opposed to the circuit-switched telephony used by the traditional Public Switched Telephone Network (PSTN). Also known as VoIP phone systems, IP telephone systems typically comprise a VoIP private branch exchange (an IP PBX) and desktop VoIP phones (IP Phones) that connect to a VoIP service provider via a Local Area Network (LAN).
One of the key advantages to IP telephone systems is that whereas long-distance calls on a regular phone system can be cost prohibitive, the same calls on an IP phone system are free -- there are no fees beyond the cost of Internet access.
See The Difference Between VoIP and PSTN Systems in the Did You Know... section of Webopedia.
Also see the VoIP Meets Wi-Fi article in the Did You Know? section of Webopedia.


