| 摘要 |
PURPOSE:To attain the talking surely by detecting a busy tone incoming from a line when an opposite party is busy to open a telephone line and detecting a ring back tone incoming from the line when not busy so as to release the muting of a transmission signal. CONSTITUTION:A control means 11 is provided with a control section 11-1, a call tone detection means 11-2 analyzing the output of a waveform shaping circuit 15 and detecting a call tone sent from a telephone line 1, a busy tone detection means 11-3 detecting a busy tone and a ring back tone detection means 11-4 detecting a ring back tone and a microcomputer. When the line is congested and the time to be connected to the opposite party is long, neither the busy tone nor the ring back tone might be detected. The control means 11 supervises whether or not a power switch 10 is depressed and controls a line close means 2 when depressed to close the line 1. Thus, sure talking is attained regardless of the length of time to be connected to the opposite party after dialing.
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