VoIP (Voice over Internet Protocol) aka Internet Telephony

VoIP is a service delivered via a combination of hardware and software. It enables people to use the Internet as a means of transmitting voice, telephone calls, by sending voice data in IP packets rather than by traditional circuit switched transmissions (PSTN.)

VoIP calls over the Internet are usually free when the calls are made between two PCs or other types of IP hardware connected directly to the Internet.

In most cases where a VoIP call is terminated to a traditional PSTN network, a small fee is charged to cover the cost of delivering the last leg of the call. Typically these costs are minimal and usually equate to the cost of a local, domestic call, given that the Internet has been used to carry the call almost all the way.