Public phones work off the standard -48V telephone supply, but need additional data delivered. One is a metering pulse of -60V applied between both lines and the earth connection of the phone. There is also a swap of line polarity when the call is connected to the other party, to tell the phone that billing has started and it must expect a billing pulse within a local unit time. If the pulse does not come and it is calling to a number not in the preprogrammed prefix table for freecall number blocks the phone will cut the call and go into tamper mode, where it disconnects, releases the line, tries to sieze the line then dials the self report number of the Telco and reports the tamper for repair. It will either stay in this mode and only receive calls, or will reset on the next successful meter pulse, depending on the programming.
Older call boxes had a much simpler mechanism, where coin data was transferred by pulses to earth when you put coins in the mechanism, which had a coil for each unit, and gave a pulse for each. Thus a 5c piece would give one pulse, a 10c 2 and a 20c 4. Even worked via an operator connected call, where the operator would listen to the clicks and count them then connect your call. Hard to fool the operator with a long tikkie, as there was a written called number record to grill. Automatic exchanges however could be fooled by wet toilet paper across the 3 wires, and dial a 0 to give 10 pulses.......... Or a 4k7 resistor on 2 croc clips, not that I ever did this, of course
, though one box had a large count on the meter and 5c in the coin box............. One guy did a 11 hour call to his girlfriend, and the conversation from his side was " Ja liefie.............Nee Liefie.................Rerig liefie" ( Yes dear, no dear, really dear) for the 11 hours..............