You need to tell the ISP that there is an external NTE and no internal master socket, so ideally you need an engineer install.
There's a high chance the first person you speak to won't have a clue what you are talking about, as external NTEs were a stupid invention that was used for a few years on new build houses then abandoned for precisely this sort of reason.
However, looking at your photograph is should be possible, (and legal), for you to bodge it without much problem. Will you need any more than one socket in the house working? If not, which one do you need? The first one after the external NTE is definitely the best to use
. I'll continue on that basis. If you want the modem/router at another, discuss it here before going any further.
The most important connections on that faceplate are those on terminals 2 and 5. Those are in fact all that is needed in most cases. I'll come back to the others later.
While that dial tone exists you need to hunt down all the extensions there are in the house. Check they all have a dial-tone.
Assuming they do, then you need to (gently
disconnect either T2 or T5 at the faceplate you have open, making sure the bare wires you now have don't make a contact with anything else, and go round checking again. If all of them have now lost dial-tone, we are in business. Though it could take a while.
As it's probable that either that socket or the other downstairs one is the first, then check the other downstairs one first. If that is off, continue checking all others. If it is still on then reconnect whatever you disconnected and disconnect the same at this other socket and repeat the "all others" test. Including the one you disconnected first, so we can be sure one feeds the other, not both fed from outside.
With luck we have now identified the "first" socket. At that remove the other of T2 or T5 and work out which T2 and T5 wires go back to each of the two incoming cables. Make sure you don't let any wire ends touch, or even touch another terminal
while you do this.
Now connect a blue/white from one pair to T2 and the white/blue of the same pair to T5. Do you get a dial-tone. If yes, job largely done. If not swap those for the other pair and check again. That should give a dial-tone. If it doesn't you have probably got the pairing wrong.
T3 is the obsolescent bell/ring wire useful only for some legacy old-type phones and a few substandard modern ones. Not needed anyway if you have a filter or a filtered faceplate. T4 is doing nothing. It has simply been used to park the pairs from T3 out of the way. I recommend you disconnect both T3 (which can cause noise on the line if left connected) and T4 and coil them out of harms way along with the now disconnected ongoing blue/white white/blue pair that fed the daisychain. Do not cut them off
At this you have a single socket working and need a dangly filter if you wish to use a landline phone. There are ways of converting to a filtered faceplate at this point, but that's another story.
That all sounds complex but is fairly simple. I'd still recommend getting the ISP to arrange an engineer install, where a pukka NTE5C and Mk4 faceplate would be fitted, possibly wherever you choose, nowhere near any existing socket.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
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