Internet telephony is the transport of telephone calls over the Internet. The process of supporting voice calls over the Internet using the Internet communications protocol IP, is known as Voice over IP (VoIP). The first business customer implementations of VoIP were long distance calls over an IP WAN as an alternative to traditional PSTN trunk facilities. Network gateway servers had converted PBX TDM/PCM signals into IP format for transport through a router network. VoIP offered no new features or functions, merely an alternative means of transport. VoIP was not used for station-to-station on-premises calls, and IP control signaling was not used for call set-up or feature activation. As the new technology evolved to include PBX system desktop call control signaling and communications over an IP network infrastructure, ToIP gradually started to replace VoIP. VoIP is still the most commonly used term, although ToIP is being used more to describe the workings of an IP-PBX system.

There are several sets of VoIP communications protocols that can be used by an IP-PBX system for call control signaling and communications transmission. Circuit switched PBXs were based on proprietary call control signaling to the digital desktops and used TDM/PCM standards for transmission across internal switching networks and to network with other communications systems over PSTN trunk carrier facilities. It was the intention of the early IP-PBX system developers to use open standard communications protocols over packet switched networks as a counter to the closed, proprietary nature of circuit switched PBXs. The communications protocol of...