Adding additional telephone sockets

Adding telephone sockets is very easy so that you can use a phone in almost any room. You can buy the individual parts you need for the work or kits which will contain the relevant parts (i.e. cable with adapter converter, wall sockets and cable clips etc.) are available. NOTE that sockets for extensions need to be 'slave' sockets rather than 'master' sockets.

It is important that telephone extension sockets and cabling are added in such a way that they do not affect the public telephone network. The telephone service provider is entitled to check and test any socket and wiring connected to the exchange lines.

Each telephone exchange line coming into a home or office should be fitted with a special 'master socket' which must be installed by telephone service provider - some older houses may still have the telephone 'hard wired' - if you don't have a 'plug-in' telephone, you will need to get your telephone service provider to install one before you can start adding telephone sockets yourself. It is Illegal to tamper with the master socket and you cannot install one yourself or wire directly to it.

Planning the installation

There are two possible basic circuits for installing additional telephone sockets:

Telephone extension circuit
  • Spur wiring - where the adapter connected to the master socket goes to a junction box and then each socket is wired from the junction box (as right).

  • Series wiring where the adapter connected to the master socket is wired to the first socket, which is wired to the second socket, which is wired to the third socket and so on (as below).

Telephone extension circuit

The choice of which is used will depend upon the position of the master socket and the required positions for the new sockets. You can mix the two forms as necessary, possibly using serial wiring to connect junction boxes on each floor, and then running spurs from each junction box to the socket positions on each floor.

A few basic guidelines are:

  1. No new socket should have more than 50 metres of cable between it and the master socket.
  2. No matter how many sockets are on a telephone line, the number of actual telephone handsets which can be connected to it is limited to 4 REN (ringer equivalence numbers) - each handset will have a REN, and the total of all the handsets must not exceed 4 REN. If any equipment is not marked with its 'REN', then check with supplier. If this is not possible, check that the equipment is approved for connection to the public telephone network (i.e. it carries a green circle on a label) and then assume a 'REN' of one.
  3. Do not position extension sockets or junction boxes out of doors or where damp or condensation may occur (e.g. shower or bath room).
  4. Only join telephone wiring within sockets or junction boxes. If a 'spur" type layout is necessary, a junction box is essential.
  5. Phone cable and sockets should be at least 2 inches from any mains electrical wiring and outlets to avoid electrical interference.
  6. ALWAYS disconnect the adapter from the master socket before changing or adding wiring to the telephone circuit.

Fixing the sockets

The first thing is to decide upon the required positions for the sockets/junction boxes. Once decided upon, offer up the boxes and as appropriate depending upon the type of wall construction, temporarily secure the boxes to the wall.

Installing the cable

Starting at the master socket, and allowing enough cable to connect the adapter to it, run the cable to the junction box or the first socket. The cable can be run:

  • On the surface, simply using cable staples every 300mm or so. It is often easier to run it along the top of skirting and around door frames, this ensures the cable is unobtrusive.
  • In surface mounted conduit, usually small profile square conduit with a removable front cover. Simply route and fix the conduit as necessary, feed the cable along it and fit the front cover of the conduit.
  • Through conduit within the wall, it normally takes a bit of forward planning to put the conduit in place before you run the cable. It can however always be retrospectively installed the next time the room is decorated, just ensure that the necessary length of cable is available. When hiding conduit in walls, always run it vertically (up or down) from the socket.
  • Through roof, floor or wall voids:
    • In the roof, keep the cable away from other electrical wiring, you can 'snake' the cable over/under roof installation.
    • Under wooden floors, the joists may need to be drilled to run the cables, again keep the telephone cables away from other electrical wiring.
    • Wall voids are generally stud partitioning or similar, normally it easier to run cables within walls during their construction.

Where cables need to go between rooms, this can be done by a number of ways:

  • Drilling through a door frame.
  • Drilling through the wall either just above the skirting board or at the top of a door frame.
  • Taking the cable either below the floor or above the ceiling and through a hole in the wall and then down or up as appropriate within the next room.

Sockets/Junction boxes

When the cable has been run to the junction box/ socket:

  • Remove the cable entry/exit points from the back plate of the box.
  • Pass the cable through the back plate from the rear, to the front.
  • Securely mount the box in the required position.

Looking at the terminations on the socket, there are normally 6 (numbered 1 to 6) terminals in the socket or junction box, the wiring colour code is:

terminal number
wiring colour code
green + white
blue + white
orange + white
white + orange
white + blue
white + green

There's no need to know what each wire does, just to know that the wiring is always the same colour to the same colour whether the cable comes from a master socket or goes to another socket.

For domestic installations, four core cable may be used, in this case these will be 2 to 5 above. If 6 core cable is available, the green and white cores can be cut back and not used.

The cable used for extending telephone systems is only designed to be used for fixed wiring to telephone socket or junction boxes, it is not designed to make extension leads or to have BT plugs fitted to it.

Two types of socket/junction box termination are available:

  1. Screw connection
    • screw connection telephone socketCarefully strip off the outer sheath from the cable without cutting the inner insulations.
    • Strip back the inner cores and make the connections according to colour code.
    • In a series circuit, two cables may enter the socket box and each terminal will have two inner cores (same colours) connected.
    • Leave slack in the wire in case it has to be removed and the connection remade.
    • Always use a cable tie to firmly secure the outer insulation to the socket/junction box cover.
    • Ensuring that the socket is correctly positioned (the cover over the socket should need to be pushed up to connect a telephone), replace the cover.
  2. IDC (insulation displacement connectors).
    insulation displacement connectors telephone socket
    insulation displacement connector tool
    • Carefully strip off about 50mm of the outer sheath from the cable without cutting the inner insulations.
    • Position an inner core (with its insulation in position) over the respectively numbered connector.
    • Use the special tool (right) to push the inner core into the connector.
    • Repeat for the remaining inner cores.
    • In a series circuit, two cables may enter the socket box and each terminal will have two inner cores connected, each core being inserted individually.
    • Leave slack in the wire in case it has to be removed and the connection remade.
    • Always use a cable tie to firmly secure the outer insulation to the socket/terminal box.
    • Ensuring that the socket is correctly positioned (the cover over the socket should need to be pushed up to connect a telephone), replace the cover.


First of all, with the adapter unconnected, use the original socket to make sure that your phone can make outgoing and incoming calls.

Repeat the test with the adapter plugged in to the master socket and the phone plugged into the adapter.

If all is working well, move along the new cable run and check each new socket in the same way.

If you have more than one telephone handset, leave the first in place and repeat the test in the first extension socket - ensure that when the bell rings, both phones operate together.

Add further handsets one at a time and repeat the test (up to a total of 4 REN loading). If you need to test more sockets than you have handsets or you reach the 4 REN loading, move the last handset from socket to socket until all sockets have been tested.

Solving problems

Two sorts of problems may be encountered:

  1. When you plug the adapter into the the master socket, nothing works. This probably indicates a short circuit in the wiring.
    • If you have an electrical tester, unplug the adapter and check out the terminals for short circuits.
    • If you cannot find the problem, starting at the first socket or junction box after the master socket, remove the wires going into the socket and retest at the adapter - if the problem persists, the problem is in the adapter.
  2. The handset works when plugged into the adapter but a socket further down the circuit does not work. This probably indicates an open circuit or poorly made connection.
    • Unplug the adapter and check the wiring within the appropriate socket and the last socket/junction boxes in the chain.
    • Check that the colours are in the correct places and that the wires are fully in position.
    • If in doubt, remake the connections.



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