Please beware that telecoms regulations and equipment change over the years.  This page covers the years 1980 to 2000.

Around 1979 Post Office Telecommunications introduced the Plug and Socket (PST) method of connection.  This document explains how the concept works.

700 type telephones always had their bells or tone ringers connected in series (max of four) and this system needed four or five wires to each phone as the bell circuit, itself, used two wires.  Each bell ringer was 1000 ohms.

PST changed this by making telephones work in a three wire system that allowed for the disconnection or reconnection of phones without causing interruption of the ringing circuit.  The Plan 4 (old style plug and socket - Plug No. 420 ) arrangement involved the use of special sockets and specially wired telephones when more than one telephone was used.

PST changed all this in so much that telephones could be removed without interfering with other telephones.

The system starts with a master socket and this contains a capacitor (dc blocking for the bell circuit), a resistor (allows the network provider to test the line and ascertain whether the line is alright even though every telephone may be unplugged) and a gas discharge tube (transient voltage surge protector). The resistance of each telephone bell is now 4000 ohms.

On a public network the master socket is owned by the network provider.  The householder can legally wire from this socket if it is the NTE5 type.  This is the socket where the front plate is removable to allow connection to the internal cable.

Every additional socket is an secondary/extension socket and contains no electrical components whatever (there can be as many extension as one wants, as long as they do not leave the building).  The extension sockets are wired to the master socket in parallel.

The sockets are wired in series with each other, effectively joining terminals 2 to 2, 3 to 3, 4 to 4 and 5 to 5 (using 6 or 4 wire cable).

The original PST sockets were grey oblong sockets with screw terminal connections and called Jack No. 620.  The white sockets appeared when the system was nationally introduced to households.  Click here for more information

Wiring Overview

The connections on the sockets are called Insulation Displacement Connectors (IDC) and are a friction connector.  The wires, complete with insulation, are forced into the IDC slot using a special wire insertion tool (Inserter Wire No. 2a).  The picture to the right shows the correct way to terminate a wire - note the wire is NOT stripped.  Do not connect more than 2 wires to each connector (the third will normally fall off as you screw the face plate back on!).

Tip:  Cut back the cable sheath about one inch and locate a nylon draw string.  Pull this draw string and split the cable sheath for about five inches and then remove the sheath exposing the wires.  Fix the cable to the socket using the cable strap supplied, then wrap the wires a couple of times around the socket housing in the middle of the socket, then fan out the wires and finally terminate the wires.  This leaves some slack in the wiring in case of maintenance.

Only use white sheathed telephone cable for internal use - black sheathed should be used externally.

The wiring should be connected to socket terminals using the following colours:-

2 - Blue/white
3 - Orange/white
4 - White/orange
5 - White/blue

If six wire cable is used the Green wires can be connected as follows (these are not used but should be terminated):

1 - Green/white
6 - White/green

If an extension bell is fitted then this must be a 4000 ohm magneto type and should be wired to 3 and 5 on any socket.

Each telephone cable is terminated with a Plug No. 431A which has a latch on the right.  Some system phones have a latch on the left to stop people inadvertently plugging them into non-system extension sockets.  The telephone lead is flat so as to fit into the plug.  A special tool is required to connect the plug and cable together.  Plug No. 431A is used with 4 wire cordage, whilst Plug No. 631A is used with 6 wire cordage.

Do not attempt to connect the old style round telephone lead to a Plug No. 431A or 631A.

BT supplied cords came in differing styles.  The common ones are tabled below.  For information on non BT line cords click here .

Cord 4/500 Straight cord Plug ended at both ends Used on Ambassador telephones, etc
Cord 4/502 Straight cord plug ended at one end and spade connections at other Used on telephones with screw terminations
Cord 4/503 Curly cord with plug at one end and spade connections at other Used on Ambassador handsets

This is the normal colour code on BT supplied cords (Cord 4/502 shown).

For more information on how line jacks work click here 

The following modification methods are for reference only and should not be used on any telephone that is to be connected to the public telephone system.
The bells would have to be changed to 4000 ohm types or a 3.3K ohm resistor fitted in series with one of the bell leads and the line cord replaced.

All telephones must now have 4000 ohm ringers - a mix of  ringers can really cause problems.

Click here for instructions on how to convert BPO telephones.
The modification instructions assume that the telephone is wired as per the relevant standard diagram.

For information on special range and other telephones click here
For information on how the PST system works click here
For information on Ringer Equivalence Numbers click here
For information on lightning protection click here
For information on how to wire telephones to Cat 5 or RJ45 socket installations click here
For information on how to convert BPO phones to Plug and Socket click here
For information on where to get telephone leads for converting your telephone click here

BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: April 18, 2021