PURPOSE: To discriminate a free line by an internal function of an exchange without controlling a terminal (subscriber's telephone set), by providing the free line subscriber's terminal with an out-band signal sending function, and detecting the response of a terminating subscriber by an incoming trunk.
CONSTITUTION: When the free line subscriber's terminal of a talkie device TK is terminated from an incoming trunk ICT, both the subscribers are connected through a switch frame SWF by the known means. When detecting that the incoming trunk ICT has discriminated the terminating to the free line subscriber from the subscriber's response status by the known method, a controlling device CONT connects the incoming trunk ICT with an out-band signal receiver REC through an incoming trunk connector IC. When confirming that an out-band signal is not sent from the terminating terminal, the controlling device CONT commonds the sending of the out-band signal to an out-band signal generator OSC. The out-band signal is sent to all the free line subscriber's terminals through jumpers on a distributing frame MDF.
HISAMURA MAKOTO
MIZUNO TADASHI
OOOKA TADAO
NIPPON TELEGRAPH & TELEPHONE