A self-contained, pre-pay coin operated telephone wherein when the user removes a handset from a switchhook, a local dial tone is generated, prompting the user to enter a telephone number of a called party. Once the first digit of that telephone number has been entered, the locally generated dial tone...http://www.google.fr/patents/US4768227?utm_source=gb-gplus-shareBrevet US4768227 - Coil operated telephone
