An auditor system (10) includes a control circuit (30) that employs an off-hook detector (32) connected across the tip and ring lines (12 and 14) of a coin telephone to determine whether the coin telephone is off hook. It also employs a step-pulse detector (36) connected across the step relay terminals...http://www.google.fr/patents/US4599492?utm_source=gb-gplus-shareBrevet US4599492 - Coin-telephone auditor
