Key telephone system

A key telephone system is a small telephone exchange system for relaying telephone communication between extensions or between an extension and an outside phone. The key telephone system consists of a main device, telephone sets, lines connecting the main device with the telephone sets and an additional unit for operating the system more efficiently. A key telephone system includes a plurality of keyphones, and an apparatus for connecting the keyphones. Each keyphone includes a line selection button for commonly using a telephone line. In a key telephone system, the communication between main and terminal devices is performed in such a manner that a key code is transmitted from the terminal device to the main device, or a control instruction code is transmitted from the main device to the terminal device. A computer communication interface is included in the terminal device. A digital key telephone system comprises a main unit connected to a public network or a private branch exchange and a plurality of digital key telephones connected to the extension lines of the main unit. A digital key telephone system performs data transfer on a data channel (Dch) based on ping-pong transmission between the key telephone unit and a key telephone set. The digital key telephone has a plurality of push button type keys. When these keys are depressed, calling the public network or the private branch exchange, answering a call, and speech communication between digital key telephones are allowed.