A method and apparatus for the detection and prevention of pin fraud in a coin operated telephone is disclosed. A circuit detects loss of loop current and triggers the opening of a pair of switches thereby disconnecting the microphone from the phone. The A-relay is also switched to its refund and collect...http://www.google.co.uk/patents/US4989239?utm_source=gb-gplus-sharePatent US4989239 - Method and apparatus for preventing pay telephone fraud and reducing handset vandalism
