The GPO box will probably only be a junction box, not the master socket.
This is because all old style GPO jacks or hardwiring of telephones has been replaced with the modern BT plug and socket system in the early 80s. If your using a modern telephones with BT plugs then your wiring has already been converted in the past and so would be considered upto BT's standards.
I have not been around long enough to know the GPO stuff in detail, but it sounds like a block 52 which is simply a 4-way inline joining block. The white, orange, black and green wires are the usable wires in the external cable entering your home. The yellow wires are for strain relief.
From the GPO box you should be able to trace an internal cable from it to a socket which will be your master socket.
Anything up to and including an identifiable socket is the BT line and BT's property and strictly shouldn't be altered.
It seems that BT do not replace master sockets with NTE5 lineboxes for free. Some engineers do it if a fault has been reported as often they can get damp and corrode after ten or fifteen years. Your best to call 0800 800 150 and ask customer service about it.