sounds like your 'master' is not the master socket just an NTE5 being used as an extension.
I would agree with Imjolly. I bet if you took the backplate off the NTE there would be nothing connected to the A & B terminals.
Quite a common thing to do, have done it myself on a few occasions when swapping a master and extension round on a FTTC provision if the NTE is flush mounted.
Do you have any other NTE's in the house, including the attic?