VoIP (Voice over IP - that is, voice delivered using the Internet Protocol) is a term used in IP telephony for a set of facilities for managing the delivery of voice information using the Internet Protocol (IP). In general, this means sending voice information in digital form in discrete rather than in the traditional circuit-committed protocols of the public switched telephone network (PSTN).
A major advantage of VoIP and Internet telephony is that it avoids the tolls charged by ordinary telephone serviceman addition to IP, over a public network VoIP uses the real-time protocol (RTP) to help ensure that packets get delivered in a timely way. Using VoIP, an enterprise positions a "VoIP device" at a gateway. The gateway receives packetized voice transmissions from users within the company and then routes them to other parts of its intranet (local area or wide area network) or, using a T-carrier system or interface, sends them over the public switched telephone network.