in the uk, a 1000 hz tone for 200 ms represents a 10p coin, and 1000 hz for 350 ms represents a 50p coin. prior to this system, the earliest uk pay-on-answer payphones used a resistance, inserted into the loop for one or several short periods followed by a short flash (complete disconnection), to signal units of money inserted. phreaks simulated these signals by unscrewing the microphone cover of the handset and inserting into the microphone circuit a resistor in parallel with a press-to-open push-button, getting the timing correct was difficult and failure to do this would result in the relay set controlling the call at the exchange to release, dropping the call. this was hard to do inconspicuously in outdoor payphones, and was more common indoors (e.g., in student halls of residence). manipulating a telephone in this way was obviously illegal and carried the specific offence of "abstracting electricity" from the gpo (later the post office and later still bt.)