Telephone

BT engineers use the following colour codes for wiring telephone extensions (though DIY engineers may do it differently) :

Pin 1 Not connected.

Pin 2 Blue with white rings (very old cable used to be a solid blue colour).

Pin 3 Orange with white rings (very old cable used to be a solid brown colour).

Pin 4 White with orange rings (very old cable used to be a solid green colour).

Pin 6 White with blue rings (very old cable used to be a solid orange colour).

Pin 6 Not connected.

In a normal household, pins numbered 1 and 6 in phone sockets are not used, so you only need 4 core cable for extensions.

Incoming BT phone cable usually has two colours :

Orange is called the “B” wire and is normally connected to either pin 2 in the master socket or if it is the newer NTE5 style socket (shown below), its own B terminal. If you measure the Voltage with a multimeter, it should be about minus 50 Volts DC with respect to earth.

White is called the “A” wire and is normally connected to either pin 5 in the master socket or if it's the newer NTE5 style socket, its own A terminal. If you measure the Voltage with a multimeter, it should be about the same as earth.

You should also be able to measure 48-50 Volts between "A" and "B".

Be careful with your phone wiring, if you make a mess of things your phone company can (and probably will) charge £100+ for a callout.