longer used due to negative effects on VDSL speeds. The socket includes a 1.8 µF capacitor (bell circuit) to feed the AC ringing and a 470 kΩ resistor (R1, out-of-service resistor) to permit remote testing when no telephones are plugged into any sockets. Additional internal extension (secondary) sockets are wired off the master socket (connected in parallel using the IDC system) but not containing the surge protector, bell circuit capacitor, and the out-of-service resistor. The 'old style' fixed master socket had only one set of terminals on the back and customers were supposed to use extension kits plugged into the front socket; however, many customers hard-wired their own extensions anyway for neatness and robustness reasons, which was a poor arrangement since it provided no way to isolate the customer's internal extension wiring from BT's wiring. The BT NTE 5 Linebox Socket allows easy disconnection of the internal wiring by removing the two screws and sliding out the bottom half of the socket. In recent years NTE5 sockets have been fitted in place of master sockets. These have a front plate where the lower half is removable so allowing customer's access to the terminals required for connecting internal extension sockets; it also provides access to a test jack, to determine if line faults are due to the customer's wiring or BT's. The removable panel also allows the external telephone line to be easily disconnected from the internal wiring, provided the wiring of the premises has been correctly carried out. The terminals on the back part used to be large screw terminals; however, all BT NTE5 sockets now use insulation-displacement connectors (IDCs). Now that BT does not have a monopoly of internal wiring, they make a substantial charge if a fault reported to them turns out to be in the customer's internal/domestic wiring. It is therefore important for the customer to have the facility to check whether any problem or fault is in their internal wiring/equipment or externally in BT's cabling or systems. Since the NTE5 socket represents the official demarcation point between the internal/domestic wiring (at the removable front of the socket which is the customer's responsibility) and the external telephone line/cabling fixed at the rear (which is BT's responsibility) the physical disconnection of the two sets of wires (made possible by the NTE5's removable front plate) is crucial in identifying faults and allocating responsibility for their rectification. In 2009, BT