UK Telephone Extension Wiring

This article explains how to wire a UK telephone extension.  You are allowed to install extension wiring so long as the connection to the master socket is via  a plug, or in the case of the master socket with a detachable lower half, via the connectors on that section.  If you decide to connect directly to the master socket or install/replace a master socket you should be aware that you are in breach of your licence to use the phone service and  your service provider may charge if they have to come to rectify a fault caused by your efforts (it is rare for them so to do unless you have been really ham fisted).

NEVER connect un-approved devices which take power from the mains to the telephone system.  Even if you survive the mains on the cabling the poor wireman sitting in a muddy hole some way away trying to find a fault may not.

Master Socket

Master Socket

This is what a typical master socket looks like.  You must have one of these at the point at which BT wiring ends and yours begins.

Master sockets should not normally be used for extensions.

Slave Socket

Extension (Slave) Socket.

All extension sockets are like this.   Note absence of ringing capacitor and surge arrrester.

The connectors shown are Insulation displacement types (IDC),  Extension sockets (especially if bought in DIY sheds) often have screw terminals.

Fixed Upper section

This is the BT supplied NTE5 master socket found in many installations, this particular one is made by Austin Taylor (  Internally it is functionally similar to the master socket above but the components are protected by a plastic cover.

User removable lower section

When the front screws are removed the complete lower portion of the socket including the IDC connectors for user wiring can be pulled out.  They connect to the back plate by means of  a fixed plug.  The fixed plug is a standard BT plug and socket and allows all the internal wiring to be easily isolated for fault finding.  A normal phone can be plugged directly into the socket remaining in the wall to test if the line is OK.

The Wiring

Cct diagram

Wiring inside a master socket

wiring disagram

Although pin 4 has no function in a domestic installation it is usually connected for the sake of neatness.  The quality of wiring carried out by many older BT wiremen is often approaching art in its perfection.

The BT Drop Cable (The cable coming from the outside world)

This often has Orange, White, Green and Black wires.  Usually (but not always) Orange and White are the active pair and go to connections 2 and 5.  In some master boxes (such as the type with a removable front section) they go to two connectors marked A and B. 

Which way around they are connected usually doesn't matter but as Rick Hughes kindly pointed out some modems (especially older USA sourced ones) and some answering machines are fussy about polarity, so it's wise if possible to check the voltage on the line and connect -48V to the B leg (J2) and 0V to the A leg (J5).

If you have underground wiring with a small grey connection box by the door the internal cabling will usually be the same type and colour as the extension cabling.

Internal Cabling

Conn No.

Usual Cable Colour




No connection


Blue with White Bands

Speech and Ringing


Orange with White Bands



White with Orange Bands

Not used but usually connected for neatness


White with Blue Bands

Speech and Ringing



No connection

An Important Note on Colour Codes

The colour code shown above is the one which would normally be used by BT.  HOWEVER it isn't always adhered to, especially if internal wiring in a new house has been installed by an electrician or it's been done or modified by  previous occupants.

You should never rely solely upon the colour code - always check both ends of the cable.

The Nice Thing About Standards Is That There Are So Many To Choose From

Just when you thought you understood it all, a brief word on plugs.  The standard plugs encountered are the small RJ11 found on line cords at the telephone or modem end and the BT431A plug at the socket end. On RJ11 it is usual for the centre two pins to carry the signal, on BT431A it is the outer two.

By a masterpiece of technical superiority between them the BSI and BT managed to number the 431A plug in the British Standard as a mirror image of the socket, so when inserted pin 1 on the plug goes to pin 6 on the socket, pin 2 to pin 5 and so on. 

As the pin numbering isn't shown on the plug you might ask why I'm confusing the issue.  Simply, many packets of plugs in the DIY sheds come with a little diagram on the back which often follows the BS numbering.  If you do the logical thing and put a plug on assuming the numbers are as on the socket the phone may not ring (and, as Adrian Graham who asked for this section to be included found out,  without knowing the numbering is askew much pulling of hair, weeping and gnashing of teeth can occur as you try to find the fault).

Looking at the plug with the contacts uppermost the BT numbering is shown above with some common colour codes.  That on the left is the older BT colour code, that in the centre is the one more commonly seen now, and that on the right  is often found on modems, imported phones and Sky/On digital boxes with only two connectors.

A simple way of remembering the correct orientation for the BT plugs  if you like mnemonics is Ring on the R ight.  When you are holding the plug as shown in the diagram with the latch on the right the ring terminal (4 on the plug, 3 on the socket) is always just right of centre.


 Note that with the two rightmost 2 wire examples in the diagram going from RJ11 to BT will involve the colour code going "wrong" at one end or the other unless a junction box or joint is used.  If the lead has a BT adapter at one end the colour swap is usually made there, USR tended to swap it in the RJ11 plug, others do their own thing.  NORMALLY the center two pins on the RJ11 are connected to the outer two (2 and 5) at the BT end.  No matter what coloured wire is used the two wires always go to pins 2 and 5 at the BT end however it isn't safe to assume they ALWAYS go to the center two at the RJ11 end.

This page was getting rather large so he sections on foreign phones and modems and Installation and Fault Finding have been moved to their own pages, please click on the links to go to them.

If you want to find an encyclopaedic site with information on almost every BT phone ever made and some good background material then have a look at