| 摘要 |
<p>A telephone installation includes at least two subscriber telephone devices (22A, 22B) each having tip and ring terminals (24A, B and 26A, B) for connection to tip and ring wires (14, 16) respectively of a common subscriber line of a telephone communication system (CO). When each device is in an on-hook condition the tip and ring wires are held at a DC voltage V1 and when only one device is in an off-hook condition the tip and ring wires are held at a DC voltage V2, where V2 is less than V1. A trigger switch (28) is connected between one terminal of each telephone device and the respective wire of the subscriber line. The trigger switch changes from a blocking mode to a conductive mode when subject to a DC voltage greater than V2 and remains in the conductive mode so long as a current exceeding a holding limit flows through the switch.</p> |
