PURPOSE:To surely prevent undesired ringing caused at a fault by controlling a selector so as to allow a ringer control section not to ring a bell tone till an initial pulse detection section detects a ringer data pulse. CONSTITUTION:Even when a fault takes place in an opposite transmitter 3 and the transmitter 3 sends a pseudo ringer-ON signal having only an initial leading, since no succeeding leading exists, a mask release signal of a ringer control section 52 is not produced from an initial pulse detection section 50 and a selector 51 is not thrown to the position of an oscillator 511, resulting that no ringer tone is generated from a called telephone set 6. Even when a fault takes place in the transmitter 3 being a caller, since no undesired ringer tone is produced in the called telephone set 6, an event that a bell tone of the called telephone set 6 is not stopped till the fault is restored is avoided.
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