PURPOSE: To attain talking without being troubled by a sound volume around the telephone set by allowing a sound volume adjustment circuit of a sound equipment to receive a detection signal from a telephone operating state detection circuit connecting to a telephone set circuit so as to adjust the sound volume of the acoustic equipment.

CONSTITUTION: A telephone use state detection circuit 2 is integrated in a telephone set 1 and detects the change in the operating state of the telephone set such as dialing, call reception, talking, and in non-use and sends its detection signal. On the other hand, a sound volume adjustment circuit 4 connecting to a sound volume circuit of an acoustic equipment 3 such as a television receiver receives the detection signal from the telephone use state detection circuit 2 through a radio channel or a wired line and adjusts automatically the sound volume off the acoustic equipment 3 in response to the signal information. That is, the tone volume of the acoustic equipment 3 is reduced during dialing, call reception and talking state and the original tone volume is restored after the end of talking. Thus, trouble of manual adjustment of the sound volume of a television receiver or the like at the arrival of a phone call is avoided and talking is attained without being troubled with the sound volume of the acoustic equipment around the telephone set.