The BT telephone cable is an inexpensive piece of wiring, with the actual cost of materials amounting to just a few pence per metre. There are two different types of BT extension leads -- 431A and 631A -- and, to tell the difference, count the slots with copper wires on the phone plug. The 431A uses four wires, and the 631A uses six wires.
Cut the telephone wire to the desired length. On one end of the wire strip back 5 cm of the protective sheathing without cutting the internal wires. Strip 1 cm from each wire.
Place the telephone cable plug on a table with the copper leads pointing down. The wires need to be inserted in the correct sequence. For a 431A head, insert the wires from left to right: blue/white, orange/white, white/orange and white/blue. For the 631A head the wires should inserted from left to right: green/white, blue/white, orange/white, white/orange, white/blue and white/green.
Place the telephone plug into the appropriate slot of the crimping tool and close the grips completely. Open the crimping tool and test the seal. Hold the telephone cable plug in one hand and gently pull on the telephone wire with the other. If the plug pulls out, the wires were not inserted deep enough. A plug can only be crimped once, so if the seal does not work the first time, throw the used plug away.
Repeat the process on the unfinished side of the telephone cable.