Voice over Internet Protocol (VoIP) refers to various transmission technologies used to enable voice communications over IP networks (such as the Internet). Internet telephony and IP telephony are often used synonymously with VoIP.

VoIP or Internet telephony involves various communications services (such as voice, facsimile, and voice-messaging) that are transported via the Internet rather than the public switched telephone network (PSTN).

To make an Internet telephone call, VoIP systems uses session control protocols (such as Session Initiation Protocol, or SIP) to set up and tear-down the call. The analog voice signal is converted to digital format using various compressor/decompressor programs (or codecs) and then translated into Internet Protocol (IP) packets. These packets are then transmitted over the Internet to the intended recipient system which reverses the process.