An 8-line telephone interface (8P8C) used with alarm systems to seize the line even if it is being used. The telephone line from the central office is wired to the RJ-31X jack and from the jack to the rest of the house or building.
With the alarm cable plugged in, the in and out wires are separated and directed to the alarm system so that it has control of the line at all times. If an alarm cable is not plugged in, the jack connects the in wire to the out wire for normal telephone operation. See RJ-11
and modular connector