Disclosed is a method that gives a priority service to enable a subscriber with an urgent message to call another subscriber who is already engaged in a telephone conversation, regardless of the reservation order, and sends an interrupt tone regularly to the target subscriber as a notification that he...http://www.google.com/patents/US4942601?utm_source=gb-gplus-sharePatent US4942601 - Method of making a call reservation and sending an interrupt tone in a private branch exchange (PBX)
