The Master Socket is the demarcation point between your internal wiring which is your responsibiltiy and BT wirings which is theirs.
They don't like you messing around with the Master Socket, infact its illegal to do anything other than run extensions from it. If they find out you have changed it, or fiddled they can cancel your contract.
The XTE-2005 replaces the bottom half of the NTE5, so you dont invalidate your contract, and further extensions always come from the bottom half.
If you dont currently have an NTE5, BT will come and fit a new one for £30.
http://www.wark19.eclipse.co.uk/nte5a.jpg - The small from part is the bit you can change, and its where all extensions are wired from. This is the part the XTE-2005 replaces.