The Voice over Internet Protocol (VoIP) specifies the transmission and reception of audio over the Internet. A connection between caller and call recipient is established using the Session Initiation Protocol (SIP). SIP has many functions, including negotiating the codecs used during the call, transferring calls, and terminating calls.
During a peer-to-peer call, VoIP phones communicate directly over IP and stream audio directly. However, analog phones and cellular phones cannot use SIP and peer-to-peer calling. Many VoIP deployments use an Internet Protocol Private Branch Exchange (IP PBX) to serve as a bridge between a phone using IP-based calling and the Public Switched Telephone Network (PSTN). Analog phones and cell phones can connect to the PSTN. By routing the audio from a VoIP phone through an IP PBX and the PSTN, a VoIP phone can establish a call with an analog or cellular phone.