A digital bypass unit for a PBX or ACD telephone system having T1 spans provides the capability to place and answer calls on a digital T1 span in the event of a system failure or power failure. The bypass unit permits the bypass of the failed system, connecting T1 channels directly to digital telephone...http://www.google.com.tw/patents/US4972452?utm_source=gb-gplus-share專利 US4972452 - Digital bypass for telephone system
