In a cordless key telephone system for servicing separate service zones (3.sub.1, 3.sub.2), a cordless station (5) establishes a two-way radio channel with an access unit (4) of the service zone in which it is located, and constantly monitors the field intensity of the established channel and transmits...http://www.google.de/patents/US5454028?utm_source=gb-gplus-sharePatent US5454028 - Cordless key telephone system having zone switching function
