Telephone system is the classic term for voice switching equipment. In general, it is understood to mean a private switching facility to which one or more extensions are connected and which is connected to the public telephone network; hence the term private branch exchange( PBX).
The designation PBX leaves open whether it is connected to the analogtelephone network( POTS), to the ISDN network or via the IP protocol, and it says nothing about the type of switching, which can be automatic, as in the Private Automatic Branch Exchange( PABX) or computer-controlled, as in the Computerized Branch Exchange (CBX), nor about the number of connected telephones. Compared to a PBX( ISPBX), which supports various communications services, the telephone system is used exclusively for voice switching. Telephone systems have interfaces to the public network and to the extension lines.