Still chasing my tail due to the cold weather.
I wonder if you could possibly take a series of photographs, upload them to a suitable hosting site and post the link here? For the complete 'picture', I would like to see: (1) a general overview of the
NTE5/A, with the lower front face-plate removed and the wiring connected thereto visible (2) a view of the first extension socket (bedroom) (3) a view of the second extension socket (downstairs) (4) a view of your new extension socket (other bedroom).
If the socket you have purchased to use at the new extension location (the other bedroom) will fit into the backing box at the existing second extension socket (downstairs), then just swap them over. That will resolve the 'doubly terminated' pair scenario.
I recommend you obtain the following:
An
xDSL V1.0 SSFP.
A pair of
Long M3 screws, if required.
An
Openreach xDSL Extension Kit.
A
box of cable clips, if required.
A pair of
gel-crimps.
At the
NTE5/A, remove and disconnect the lower front face plate. Assuming the three-pair cable used is the green, blue and orange type, transfer the telephony extension pair (assumed to be blue) to the respective IDC2 & IDC5 of the new
xDSL V1.0 SSFP. Assuming the green pair in that cable to be unused, connect them to IDCB & IDCA (green/white stripe & white/green stripe, respectively). Use a cable-tie to ensure that the insulation of the extension cable is attached to the fixing post of the
SSFP. Don't plug it back into the
NTE5/A until the rest of the wiring is completed.
From the first telephone extension socket (bedroom), run the new data cable (cut off the plug) to where the data socket will be located in the second bedroom. At the first telephone extension socket, connect the blue pair of the data extension cable to the green pair arriving from the
SSFP. Blue/white stripe to green/white stripe and white/blue stripe to white/green stripe with a pair of gel-crimps. Neatly tuck the crimps and wires into the back of the box and refit the socket.
At the new data socket in the second bedroom, connect the blue/white stripe to IDC4 and the white/blue stripe to IDC5. (You should be using the middle two connectors of the eight.) Neatly tuck the wires back into the box and fit the socket.
To test, fit the
xDSL V1.0 SSFP to the
NTE5/A. With a classic, wired telephone plug it into the telephone socket on the
SSFP. Call
170170 to ensure that socket is working. Take the phone to each of the other two telephone extension sockets and repeat the test, in turn. Hopefully they are all working correctly. Now connect your modem/router to the data extension socket in the second bedroom and watch its lights. Ensure that the correct lights are lit, signifying that it has established sync with the DSLAM in the exchange. If at all possible, access the statistics from the modem/router and note the Sync speed, the Attenuation and the target SNRM (for both DS and US).
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For the proposed shopping list, above, I have selected the items from the
Clarity on-line site. Obviously there are other sites you could use. The choice is yours.