The System 25 is a rare PBX that used AT&T's Merlin telephones giving the customers a System 75-type of feel. Basing on it's port count and it's limitations it was a replacement for the Horizon PBX that was from Western's days in the Bell System. It was a system that shared the Information Services Architure of foot long boards and other input/output that resembled the System 75. The cards were incompatible with the larger systems, but the phones were. How many System 25s were in the wild is unclear, it's documentation goes from 1987 to 1990. It's most likely replacement was a low end System 75 (later Definity G1 and G3) and or a Legend.