| 摘要 |
PURPOSE:To facilitate the obstruction of illegal speech by extremely lowering the impedance of a trunk line when the illegal communication is intended by installing a telephone set other than the prescribed one illegally in parallel with the telephone set of a public telephone set, etc. CONSTITUTION:The telephone set is installed in parallel illegally for lines A, B while the public telephone set 10 is in an on-hook state and a continuous pulse for detection is generated from a control part 23, and when said telephone set turns into an off-hook state, pulse output from a photocoupler PC2 becomes L-level on account of a voltage drop due to the loop current of the non- prescribed telephone set. Therefore, an opening signal is outputted for a communication loop forming circuit 11 or an AC impedance lowering circuit 21 from the control part 23, and the impedance between the trunk lines falls extremely low by the impedance lowering circuit 21. Accordingly, a decadic pulse or a DTMF signal from the non-prescribed telephone set is not satisfactorily transmitted to an exchange 1, and the speech through the non-prescribed telephone set is obstructed. |
