BT engineers use the following colour codes for wiring telephone extensions (though DIY engineers may do it differently) :
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 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's the newer NTE5 style socket (shown below), it's 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, it's 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.