PURPOSE: To make it possible to detect the response of an opposite station by tones even when a response signal is not transmitted from the opposite station.

CONSTITUTION: This system is provided with a tone detection circuit 2 detecting the existence of the tone of the frequency preliminarily fixed within a central trunk 1, a tone detection circuit 3 detecting the existence of the tone other than this preliminarily fixed frequency and a means detecting the response of the subscriber of an opposite station based on the detection result of the existence of the two tones when the trunk is transmitted. The device detects tones (videotex switching instruction signal) incoming from a Captain network, etc., and defines them as the response of the opposite stations.