Have a wee nosy at these two links.
Does the flat have an active phone service?
What phone sockets are in the flat now? if you google Openreach NTE5 do any of them look like that?
This may be of interest:
and one more thing, Now only offer copper based services , Nows prices are good, but if i was starting from scratch - and didnt need a fixed line ( aka landline ) number i would look to full fibre/fttp (if available)
Now comes over a landline to that socket, but you don’t pay extra for the landline, it’s included in the quoted price for the broadband service.
But maybe what you mean here is that you don’t need it to make telephone calls on? In which case, ignore the bit where Now say you get a Pay As You Go service on it, don’t plug a phone into it, and just pretend it’s not there.