A multi handset telephone system with a wireless telecom/doorbell module. The telecom/doorbell module has similar electronics as a cordless handset that includes audio and RF link with a base unit. A speakerphone functionality of a wireless telecom/doorbell audio processor is used to provide an intercom...http://www.google.com.tw/patents/US7738917?utm_source=gb-gplus-share專利 US7738917 - Intercom/wireless door bell for multi-handset telephone system
