This is a standard BT main socket which has been gradually replacing a multitude of different GPO connectors (example shown below).  The front panel, held by two screws plugs into a socket in the back-plate. The BT line connects to the back plate socket, and any extension wires should connect to the terminals on the front panel. The idea being that if you unplug the front panel all your house telephone wiring is eliminated and the BT line can be checked.  However I have seen sockets like this where extension cables have been connected in to the back plate, which defeats the object.  [If the top half if the box is embossed OPENREACH then the bell wires will be filtered out]

Old GPO connector box