An IP PBX is a system that connects telephone extensions to the public switched telephone network and provides internal communication for a business. The IP PBX has a directory of all phones/users and their corresponding SIP address and thus is able to connect an internal call or route an external call via either a. What Is A IP PBX? Also known as a PBX, Unified Communications System or business phone system, a PBX acts as the central switching system for phone calls. This presentation shows how an IP PBX works and what its main benefits are. Also referred to as an IP PBX, VoIP PBX systems can typically switch calls between VoIP on local lines or between VoIP and traditional telephone users in the same way a PBX does. An IP PBX is a central system that switches and routes calls between the telephone network and VoIP users.