Intercom adapter for telephones

摘要

Apparatus for adapting a standard telephone set for use in an intercom system communicating over the two "extra" conductors of the four-conductor wiring typically provided on the premises of a telephone subscriber. The adapter is inserted between the telephone and the premises wiring. A hold button, intercom signaling button, intercom select switch, and various indicators are provided by the adapter, which provides these features without requiring separate connection to a power source. The hold function automatically releases at the end of a conversation. Indicators warn the user to switch from intercom to central office when an incoming call is present and the intercom is selected. The intercom is usable even when the central office line is out of order.