 Its intercommunication ability allows two or more stations to directly connect while not using the public switched telephone network.This method reduces the number of lines needed from the organization to the public switched telephone network.Each device connected to the PBX, such as a telephone, a fax machine, or a computer modem, is referred to as an extension and has a designated extension telephone number that may or may not be mapped automatically to the numbering plan of the central office and the telephone number block allocated to the PBX.