Wiring modern telephone sockets (Line Jacks)
Wiring Diagram for Master & Extension Telephone Sockets

Pin | Use | Master Socket | 1st Secondary Socket | 2nd Secondary Socket | 3rd Secondary Socket etc. | Pin |
1 | Not used § | Green/White | Green/White | Green/White | Green/White | 1 |
2 | ‘B’ wire (-50V to earth) | Blue/White | Blue/White | Blue/White | Blue/White | 2 |
3 | Bell wire | Orange/White | Orange/White | Orange/White | Orange/White | 3 |
4 | Earth (PABX use only) | White/Orange | White/Orange | White/Orange | White/Orange | 4 |
5 | ‘A’ wire (0V to earth) | White/Blue | White/Blue | White/Blue | White/Blue | 5 |
6 | Not used § | White/Green | White/Green | White/Green | White/Green | 6 |
When ‘new-style’ wiring was introduced in the early eighties a domestic single line telephone installation would have a master socket which is provided by the service provider (B.T., cable company etc.). This was usually an LJU2/1A or a LJU3/1A type of socket. A master socket contains a 1.8µF capacitor to pass ringing current to the telephones, a surge arrestor 11A or 26A to suppress high voltage spikes etc. and a 470k ohm resistor for testing purposes. Additional secondary sockets are wired off the master as shown above, the colour codes shown are the standard, but may differ in some existing installations. The wiring diagram for a master LJU socket is shown here. Note that the pictorial view shown above only serves to indicate the two differing types of sockets and wiring connections. Contrary to how it might seem, C1 is connected to terminals 2 & 3, not 1 & 4
NTE5 Line-box wiring instructions
In more recent years NTE5 or CTE5 Lineboxes are fitted in place of master sockets, these have a removable (lower half) panel which house the terminals to connect wiring to the secondary sockets. § Terminal pins 1 and 6 may be absent on some versions of C/NTE5, but these connections are not normally used anyway. The NTE5 tends to have the same components inside as a master LJU, however later NTE5 sockets differ by not having a surge arrestor, have only pins 2, 3, 4 & 5 or 2, 3 & 5 connected, they also have a small coil in series with the ‘bell wire’ on pin 3 which acts as a filter.
On older installations (pre-1980) cream or grey cables were used. They contained conductors coloured blue to pin 2, orange to pin 5, brown to pin 3 and green to pin 4 (if used), on some conversions green was used for pin 3 and brown for pin 4. On some installations of internal extension wiring (post-2012) BT/Openreach have now come full circle and have reverted back to using white four core cable containing solid colours: blue to pin 2, orange to pin 5, brown to pin 3 and green to pin 4 (if used).
Additional Bells are wired between pin 3 (orange/white) and pin 5 (white/blue).
External overhead telephone line colour code: orange / white = line 1; green / black = line 2.
Wiring Faults
Telephone line dead or similar issues


Most homes with a landline will have a master socket called an NTE5A, this socket has a concealed test socket in which you can plug in a phone to test the line.
To expose the test socket simply remove the two screws and gently pull off the front faceplate, be sure to do this gently as there may be wires connected to the back of it.
You will then see the test socket in which you can plug in a phone in order to test the line. If the line now works then it is very likely that you have a fault with your extension wiring or sockets. If the line is still dead then the likelihood is that the fault is outside your home, although it could still be a fault on your property.
The latest type of master phone socket is called an NTE5C, the same principal applies except that the front faceplate simply unclips from each side, rather than being screwed on.
Other telephone problems
Phone ringing continuously:
No ringing:
Ringing but no speech (or very poor speech) and can’t dial out:
Very poor speech quality, possibly poor bell:
Terminals 2 &5 swapped between sockets (2 at one socket connected to 5 on another & vice versa)
Terminal 3 disconnected
Wire broken between terminals 2 or 5
Terminal 3 and 2 or 3 and 5 transposed
Telephone Cable Wiring Colours
CW1311 Internal 4/6 Way Telephone Cordage
Contact 1 |
Black |
system phones |
6-way only |
|
Contact 2 |
White |
‘A’ wire |
4 & 6-way |
|
Contact 3 |
Green |
Earth wire |
4 & 6-way |
|
Contact 4 |
Blue |
Bell wire |
4 & 6-way |
|
Contact 5 |
Red |
‘B’ wire |
4 & 6-way |
|
Contact 6 |
Orange |
system phones |
6-way only |
CW1308 Internal Telephone Cable
Pair No. | ‘A’ wire | ‘B’ wire |
1 | White/Blue | Blue/White |
2 | White/Orange | Orange/White |
3 | White/Green | Green/White |
4 | White/Brown | Brown/White |
5 | White/Grey | Grey/White |
6 | Red/Blue | Blue/Red |
7 | Red/Orange | Orange/Red |
8 | Red/Green | Green/Red |
9 | Red/Brown | Brown/Red |
10 | Red/Grey | Grey/Red |
11 | Black/Blue | Blue/Black |
12 | Black/Orange | Orange/Black |
13 | Black/Green | Green/Black |
14 | Black/Brown | Brown/Black |
15 | Black/Grey | Grey/Black |
16 | Yellow/Blue | Blue/Yellow |
17 | Yellow/Orange | Orange/Yellow |
18 | Yellow/Green | Green/Yellow |
19 | Yellow/Brown | Brown/Yellow |
20 | Yellow/Grey | Grey/Yellow |
21 | Violet/Blue | Blue/Violet |
22 | Violet/Orange | Orange/Violet |
23 | Violet/Green | Green/Violet |
24 | Violet/Brown | Brown/Violet |
25 | Violet/Grey | Grey/Violet |
26 | Pink/Blue | Blue/Pink |
27 | Pink/Orange | Orange/Pink |
28 | Pink/Green | Green/Pink |
29 | Pink/Brown | Brown/Pink |
30 | Pink/Grey | Grey/Pink |
CW1293 Internal Telephone Cable
Pair No. | ‘A’ wire | ‘B’ wire |
1 | White | Blue |
2 | White | Orange |
3 | White | Green |
4 | White | Brown |
5 | White | Grey |
6 | Red | Blue |
7 | Red | Orange |
8 | Red | Green |
9 | Red | Brown |
10 | Red | Grey |
11 | Black | Blue |
12 | Black | Orange |
13 | Black | Green |
14 | Black | Brown |
15 | Black | Grey |
16 | Yellow | Blue |
17 | Yellow | Orange |
18 | Yellow | Green |
19 | Yellow | Brown |
20 | Yellow | Grey |
21 | White/Blue | Blue |
22 | White/Blue | Orange |
23 | White/Blue | Green |
24 | White/Blue | Brown |
25 | White/Blue | Grey |
26 | Red/Blue | Blue |
27 | Red/Blue | Orange |
28 | Red/Blue | Green |
29 | Red/Blue | Brown |
30 | Red/Blue | Grey |
CW1128 & CW1326 External Cable
Pair No. | ‘A’ wire | ‘B’ wire |
1 | White | Blue |
2 | White | Orange |
3 | White | Green |
4 | White | Brown |
5 | White | Grey |
6 | Red | Blue |
7 | Red | Orange |
8 | Red | Green |
9 | Red | Brown |
10 | Red | Grey |
11 | Black | Blue |
12 | Black | Orange |
13 | Black | Green |
14 | Black | Brown |
15 | Black | Grey |
16 | Yellow | Blue |
17 | Yellow | Orange |
18 | Yellow | Green |
19 | Yellow | Brown |
20 | Yellow | Grey |
CW1406, CW1411 & CW1417 External Dropwire Cable
CW1406 | CW1378 & CW1411 | CW1417 (CAD55M) | ||
1-Pair No.12 | 2-Pair No.10/10B | 4-Pair No.14 | ‘A’ wire | ‘B’ wire |
1st pair | 1st pair | 1st pair | White | Orange |
n/u | n/u | 2nd pair | Grey (slate) | Red |
n/u | n/u | 3rd pair | Brown | Blue |
n/u | 2nd pair | 4th pair | Black | Green |
Dropwire comes in 1-pair, 2-pairs or 4-pairs, the cable also has three steel tension wires to give support. |
CW1406, CW1411 & CW1417 External Dropwire Cable
Pair No. | ‘A’ wire | ‘B’ wire |
1 | White | Orange |
2 | Black | Green |
Double sheathed external grade telephone cable available in white & brick brown colours. |
||
Item code: 055354 |
Early type of external dropwire cable were constructed of one parallel pair in a figure-8 configuration.
BT dropwire reference | No. 1 | No. 2 | No. 6 | No. 8 |
CW Reference | CW1166 | CW1208 | CW1247 | CW1320A |
Conductor Type | Solid cadmium copper | Solid copper covered steel | Solid copper covered steel | Solid cadmium copper |
Nominal Conductor Diameter (mm) | 0.91 | 0.71 | 0.81 | 1.14 |
Conductor Size (mm2) | 0.65 | 0.4 | 0.52 | 1.02 |
Minimum Insulation Resistance @500V DC (MΩ.km) | 24 | 24 | 50 | 24 |
Minimum Insulation Thickness (mm) | 0.75 | 1.02 | 0.8 | 1.52 |
Nominal Insulated Conductor Diameter (mm) | 2.41 | 2.75 | 2.41 | 4.18 |
Colours for CW1109 Jumper Cable used on Main Distribution Frames (MDF) inside BT Telephone Exchanges
Application |
‘A’ wire |
‘B’ wire |
TXE4 & misc. non PSTN BT circuits | Blue | Yellow |
System X circuits | Green | Black/white |
System Y (AXE10) | Orange | Turquoise |
Private circuits | Pink | Grey |
Featurenet & Redcare circuits | Red | Yellow |
DACS & WB900 Circuits | Green | Yellow |
P wire busying | Green | – |
Jumper wire colours
CW1109 Jumper wire is available in various colours, including, red/white, red/black, blue/yellow, blue/white, green/white, green/black and pink/grey. It is used for connecting MDF’s and for Krone disconnection strips.
Note regarding telephone cable colours
When describing telephone wiring the colour grey is often referred to as slate. This is to avoid any confusion between green and grey as the first three letters are both the same.