UK telephones can have internal extensions added to them, but there are laws governing the location and what type of extension is allowed within a domestic property. The master socket, cabling and any telephone equipment leading out of a home are considered to be the phone company's responsibility and property as they connect the customer to the exchange. An extension can be installed if it is connected to the master socket via a plug. You cannot wire directly into the master socket as this breaches the service provider's agreement, and you are likely to be fined or charged a fee should a fault arise as a result of tampering with their equipment.
Unscrew the front cover of the extension box and remove the cover plate.
Determine the type of extension box you are wiring. Modern boxes have six push in connectors for the wires and are punched in using a plastic insertion tool. Older models require the wires to be inserted under screws and tightened using a screwdriver.
Strip back 1.5 inches of insulation on the outer cable to expose the colour coded internal wires. Domestic wiring uses four-core cabling and so there will be four separate wires inside.
Strip away approximately half an inch of insulation from each of the four coloured wires to expose the metal core that will form the connection when attached to the terminals.
Position each coloured wire to the corresponding markings on the terminals inside the extension box. If there are no markings, they should be matched like this:
Terminal 1: green with white rings
Terminal 2: blue with white rings
Terminal 3: orange with white rings
Terminal 4: white with orange rings
Terminals 5 and 6 will be left empty as they are not used in domestic telephone systems.
Punch each wire into the corresponding colour-coded terminal using the insertion tool, or secure each one using the screwdriver if they are screw fitting.
Replace the cover plate on the extension box and screw down securely.
Extension kits are available from DIY stores and consist of a pre-wired extension box, cable and socket plug. A kit provides a ready-made solution to adding another phone to an existing line without the need for wiring any of the internal parts. The cable is plugged into the master socket, allowing an additional handset to be connected via an extension.
UK law states that the telephone master socket must be fitted by the phone company, but extensions may be added after this up to 50 yards away from the master socket. A telephone extension should never be installed in a bathroom or shower room. Telephone cable should always run at least 2 inches away from other domestic electrical cables.