Money for the charge is not charged until the called subscriber answers the call. If you make the PBX think that the handset is not picked up, the bill will not come. To do this, it is necessary on the side of the called subscriber to keep the voltage in the line after picking up the handset so that the sending of the inductor call signals stops. The point is that for discontinuing an inductor call and turning on the counter, different relays in the station equipment are responsible, and the first relay is triggered at a higher voltage than the second (this is done so that the second does not operate falsely from the inductor signal). That is, you call someone who has this device and you do not charge money. To keep the voltage in the line, you can use the scheme:
A zener diode for a voltage of 22 ... 26 volts (selected on the spot, gradually increasing, until the inductor call stops). You can apply several series-connected zener diodes for lower voltage. When using a modem, it may sometimes be necessary to parallel the bridge diodes with 0.1 ... 1 uF capacitors. The diode bridge can be eliminated if a bi-directional zener diode is used (or a pair connected in opposite directions), or the circuit can be switched on according to the polarity of the line. Note: Freak only works on 60 volt lines. In some localities 5-volt lines have spread, this trick will not work on them.