PRIVATE LINE TALKING SYSTEM

摘要

PURPOSE:To return a ring-back tone possible for listening by using an intermittent DTMF (Dual Tone Multi Frequency) signal as a call signal to output the call signal with a large output. CONSTITUTION:When a handset of a caller side telephone set 8 is lifted, notice of hook-off is given to a telephone set control circuit 15. The circuit 15 actuates a DTM oscillator 16 and switches an analog gate 17 in a period of on (1sec) and off (2sec). Thus, the DTMF signal is outputted to a line 1a with an output of -4.5dbm. The signal acts like a call signal. When the DTMF signal is inputted to the reception side, a DTMF signal detection circuit 19 detects the signal and gives notice to a telephone control circuit 15 to switch an analog gate. An oscillator 18 makes oscillation to send a signal to the line with an output of -4.5dbm. Thus, a ring-back tone is returned.