According to this invention, a digital key telephone system connected to an analog public network NW having a function of transmitting a ringing signal including identification information of a calling line through a subscriber line (CO line), accommodating a plurality of extension lines each connected...http://www.google.com/patents/US6496576?utm_source=gb-gplus-sharePatent US6496576 - Digital key telephone system and extension terminal device thereof
Digital key telephone system and extension terminal device thereof
