Key telephone system with reduction of delay of ring tone generation at

Abstract

A plurality of trunk interface blocks are provided to receive a plurality of telephone lines. In each trunk interface block, a ringing signal detection circuit detects an incoming ringing signal on the corresponding telephone line, and a code insertion circuit inserts in a PCM signal, coded data indicative of whether or not an individual ringing signal exists on the telephone line in response to the output of the ringing signal detection circuit. The PCM signal is transferred through a PCM time division switch and an extension interface circuit to a destination extension set. In the destination extension set, the code discrimination circuit extracts coded data from the PCM signal and controls the on-off circuit to drive the speaker in synchronization with the individual ringing signals.