A telephone switching system for interconnection of a plurality of lines with a number of trunks in which telephone sets having no trunk access pushbuttons can access particular trunks upon command. Upon initiating a command signal, i.e. by a subscriber dialing particular access codes, the switching...http://www.google.es/patents/US4376875?utm_source=gb-gplus-sharePatente US4376875 - Keyless and indicatorless local telephone switching system
Keyless and indicatorless local telephone switching system
