If the cut cable was before the master socket then that is BTs property and
cannot be touched.
The master socket is BTs demarcation point.. what happens after the master socket - ie fitting extensions etc, thats up to the owner if they pay BT to do the work, or you can use the DIY method.
Touching the dropwire behind the master socket is a big NO NO. The drop wire consists of different cabling and is 2 twisted pairs (A+B) rather than the 4 strands in normal telephone extension.
From the OPs post its not 100% clear to me which part was cut.. {quote}"
now runs from the pole outside and terminates inside the house."{/quote}.
Its also illegal to replace an LJU2 master socket with an NTE5. However if the cable beyond the existing master is still intact and if its a case of wanting to wire up a "new NTE5" so that you can connect the new extension wiring.. the only way it can legally be done is by following the instructions on the bottom of
this page.
By using one of the clarity specially adapted NTE5s, you can then connect into that the ethernet cable youve laid which runs to the new extension. Theres an example how to do that on the same page.
At the risk of being repetitive but I really cannot stress this enough - you cant touch anything before the old master socket... and if that is the part thats been cut, then you will have to call out BT.