Loop line
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A loop line is a type of circuit used in telephone networks. It does not connect to a specific place, and requires two numbers dedicated to it. When one side of the loop is called, there is a tone (called Side A), and on the other there is dead silence (Side B). When a line is connected to side A, any number of lines may be connected to side B (there is usually a limit on this) and be connected into a conference with the person on A. The function of the tone on side A is so that whoever is connected will know when somebody has called the B side and thus connected.
Loop lines are far less common now than they were around 1960, however they reportedly still exist. Because of their nature, however, telephone companies work to protect them (in order to make people use conference calling). The most common methods are:
- Filters which bandstop voice (these can be switched on or off)
- Recordings which make the line appear out of service
- The loop being switched on or off by operators
- Restrictions on which lines may call
- Using tones in the 1633 column of DTMF, A-D, in the number