Actually, pay phones didn't need batteries and microcontrollers when they were operated exclusively by the telephone companies. In addition to 48V across tip an

Actually, pay phones didn't need batteries and microcontrollers when they were operated exclusively by the telephone companies. In addition to 48V across tip and ring, they used + and - 130V from ring to ground to control the operation of the coin relay. One polarity caused the coin to be collected, and the other polarity refunded the coin.