An IP PBX (phone server) is a device that controls the extensions, call routing, connections to voice over IP service providers and the Public Switched Telephone Network (PSTN) and provides many other services for a VoIP phone system. The IP PBX can be thought of as the brains of a voice over IP phone system. The IP phones, analog adapters & VoIP gateways communicate with the IP PBX to place or receive calls.

FXS interfaces are used to connect analog telephones or faxes to the IP PBX. FXO interfaces are used to connect Central Office lines (analog telephone lines from the phone company) to the IP PBX. T1 and E1 interfaces are used to connect the IP PBX to high speed, ultra-reliable internet connections..