Communication systems of key telephone systems

Claim 1

A communication system for a key telephone system wherein a key telephone is selected by a key service unit and electric power is supplied through a signal line for performing an independent call, the key service unit comprising a voice switch adapted to switch between transmission and reception of speech, each key telephone comprising a loudspeaker, a loudspeaker amplifier and answer signal sending means comprising a microphone and a microphone amplifier, the answer signal sending means being rendered operative by the power supplied from said signal line, a single common answer signal line commonly connected to the outputs of respective answer signal sending means so that when a particular key telephone is called the voice switch operates to switch between the transmission and reception of the speech in response to a call signal and an answer signal permitting the selected key telephone to perform a hand-free communication, a selection circuit disposed between the voice switch and the loudspeaker amplifier, said microphone amplifier being disposed between the microphone and the voice switch, the selection circuit enabling one signal line to select the key telephone, start the loudspeaker amplifier and start the microphone amplifier.