A key telephone system has a main service unit including at least a main controller, a terminal interface and an office line interface, and a telephone terminal which is connected to the terminal interface. The telephone terminal includes a handset for transmitting and receiving a voice or a speech signal...http://www.google.fr/patents/US5388152?utm_source=gb-gplus-shareBrevet US5388152 - Key telephone system capable of regulating volume of handset
Key telephone system capable of regulating volume of handset
