‘BT’ Telephone Wiring

dataIP Technical Information Document 18: , Last Revised : 2002-11-29

Connector number	Incoming BT Socket Cable Colours (base colour/stripe)	Telephone Cable Colours	Description
1	white/green	black	Not usually used
2	white/blue	white	Line A
3	white/orange	green	PABX earth recall
4	orange/white	blue	Bell shunt
5	blue/white	red	Line B
6	green/white	orange	Not usually used

The connector numbers shown above are the numbers you would find inside a standard BT socket. There is confusion concerning the numbering method for the telephone plug that connects to the BT socket… You need to be aware there are two different numbering schemes. The British Standard which covers Type 431A plugs labels pin 6 being at the latch side (contacts facing you), whereas BT designate this as pin 1. Some plug suppliers provide BS numbers with their wiring diagram, others give the BT numbering.

The following image shows a plug with the BS numbering scheme. Just reverse the numbers to match the BT socket numbers. The diagram assumes the contacts are facing you.

Lines A and B are often referred to as the line pair, and are all that’s needed for a modern phone to work. Line A is usually at 0 volts and B is at -48 volts DC (nominal, but can vary from -6 to -85 volts depending on your location from the exchange and what your phone is actually doing at any particular time). All modern equipment has to be designed to work if the polarity is reversed, eg if the white and red wires inside the phone are swapped around. If you look at the BS and BT numbering schemes, you’ll see that because lines A and B are symmetric, in effect it doesn’t matter which numbering scheme you use as the line pair will always be on pins 2 and 5. The only thing you need to worry about is if you want to use the Bell Shunt or PABX Earth Recall pins.

The Bell Shunt used to be used to ring the phone, but modern phones sense the ringing current from the line pair.

The PABX Earth Recall is usually used by switchboard systems.

The cable from the phone doesn’t usually have the black and orange wires, unless it’s a really old model. Some types of business and data communications hardware use these.