Money for the time period is not accrued until the called subscriber removes the call. If you make ATC think that the handset is not removed, the account will not come. To do this, it is necessary to keep the line voltage on the side of the called subscriber after the handset is removed so that the sending of the induction call signals is stopped. The point is that for the termination of the inductor call and the inclusion of the counter there are different relays in the equipment of the station, and the first relay operates at a higher voltage than the second one (made so that the second does not act falsely from the inductor signal). That is, you call someone who has this device and you do not accrue money. To keep the voltage in the line, you can use the following scheme:
Zener diode for a voltage of 22 ... 26 volts (select on the spot, gradually increasing, until the termination of the inductor call). It is possible to apply several series-connected zener diodes to a lower voltage. When using a modem, it may sometimes be necessary to parallel the bridge diodes with capacitors of 0.1 ... 1 uF. The diode bridge can be eliminated if you use a bi-directional zener diode (or pair turned on), or turn on the circuit in accordance with the polarity of the line. Note: Frick only works on 60-volt lines. In some areas 5-volt lines have spread, this trick will not work on them.