Voice over IP Telephony is the technology that makes it possible to have a telephone conversation over the Internet or a dedicated Internet Protocol (IP) network instead of dedicated voice transmission lines.

This allows the elimination of circuit switching and the associated waste of bandwidth. Instead, packet switching is used, where IP packets with voice data are sent over the network only when data needs to be sent, i.e. when a caller is talking.

The protocols used to carry the signal over the IP network are commonly referred to as Voice over IP or VoIP protocols.

Although few office environments and even fewer homes use a pure Voice over IP infrastructure, telecommunications providers routinely use IP telephony, often over a dedicated Voice over IP network, to connect between their switching stations, where they convert the dedicated voice signal to IP packets and back. The result is a data-abstracted Vocie over IP digital network which the provider can easily upgrade and use for multiple purposes.

Voice over IP is also used by large companies to eliminate call charges between their offices, by using their data network to carry inter-office calls. They may also use VoIP to reduce the costs of calls outside the company by carrying them to the nearest point on their network before handing them off to the PSTN.