an external call’s ringing signal can be heard in the earpiece. Many key telephone systems for office use provide access to multiple outside lines plus an intercom, where the latter appeared as if it were one of the individually-selectable lines. The Western Electric Model 207 and Model 6A intercoms were designed for integration into such key system multiline installations, providing one or two simultaneous intercom calls respectively. See also Bticino Courtesy phone Door phone Fermax Public address system Speaking tube Video door phone References External links List of mobile video intercoms Telecommunications equipment