A system for location of communication end users including a plurality of wide area networks which control the transmission of voice, data, file and other types of messages in a preconfigured exchange area, each area network including a plurality of local servicing offices controlled by a central regional...http://www.google.com.hk/patents/US5659596?utm_source=gb-gplus-share專利 US5659596 - System for location of communication end users
