 As PBX systems gained popularity, they began to feature services not available in the public network, such as hunt groups, call forwarding, and extension dialing.From the 1960s, a simulated PBX, known as Centrex, provided similar features from the central telephone exchange.A PBX differs from a key telephone system (KTS) in that users of a key system manually select their own outgoing lines on special telephone sets that control buttons for this purpose, while PBXs select the outgoing line automatically.