Key telephone system

Abstract

In a key telephone system with a plurality of four-wire key telephone sets wherein a two-wire telephone set is connected to a main control device via a pair of speech lines so that intercom speech and office line speech are possible between the two-wire telephone set and other key telephone sets, the improvement wherein said main control device comprises a call signal generating circuit, first transfer switching means for transfer-connecting said speech lines between a pair of office speech lines and a pair of intercom speech lines, second transfer switching means for transfer-connecting said speech lines between said call signal generating circuit and said first transfer switching means, DC loop supervisory means for supervising a DC loop of said speech lines, and a control circuit inputted with an output of said DC loop supervisory means for controlling said first and second transfer switching means in accordance with the states of said office lines. Under a normal state the speech lines are connected to the intercom speech lines through the first and second transfer switching means. At the time of arrival of an office line call signal the second transfer switching means is switched to send the call signal to the speech lines. Thereafter the second transfer switching means is reset and the first transfer switching means is switched to the office line speech lines in accordance with the output of the loop supervisory means.