FIELD: communications engineering. SUBSTANCE: time relay is inserted between calling terminal and call-shaping terminal for changing communication link in establishing call connections for detecting failure source during calling procedure. If only calling party can hear voice of called party and the latter does not hear any voice, time relay functions to connect calling-party output to his own input so as to check if he can hear his own voice. If he does, it means that failure is not in calling party. Then time relay connects output of called party to his own input to check if called party can hear his voice. If he fails to hear his voice, it means that failure source is on called-party side. EFFECT: facilitated location of source of failure by calling party in establishing communications. 2 cl, 2 dwg
