Abstract: A telephone switching system is provided for use with an external telephone line and a plurality of telephone extensions wherein each extension has a plurality of "touch tone" buttons which, when depressed, generate an audio signal unique to that button. The system includes at least one internal channel and at least one external channel wherein the external telephone line is connected to the external channel. An on-hook and off-hook detector is provided for each of the telephone extensions as well as a detector for detecting the presence of an incoming call on the external channel.