When you have broadband/ADSL problems the first thing your internet provider asks you to do is to "try the test socket". Often they don't explain why - or they may just say try the "master socket" which is a slightly different thing.
Why use the BT TEST socket?
Internal Phone Wiring - Phone Extensions
Only BT are legally allowed to take the Master Socket apart further than this - householders can only remove the lower faceplate!
BT 'TEST' Socket - NTE5
NTE5 - Network Terminating Equipment
I'm connected via the BT TEST socket and my
broadband speeds haven't changed!
haven't got a modern Master Socket - NTE5!
If you contact BT and ask for your "phone wiring to be
regularised" or say you are unable to fit a new phone extension
because your don't have a modern Master Socket then BT should come and fit
an NTE5 for free.
Fitting a BT iPlate to your Master Socket