A key telephone system having an off-hook announcing function comprises a main unit and a plurality of standard key telephone sets each connected to the main unit. In the system, the main unit includes a plurality of digital telephone interface circuits connected to the key telephone sets and also a...http://www.google.com/patents/US5128989?utm_source=gb-gplus-sharePatent US5128989 - Key telephone system with circuitry for adding a third caller to a multiple party connection
