Is there any noise on your phone calls? Dial 17070 and select option 2, there should be no noise between the announcements.
I have just tried this with the landline and there seems to be a quiet little fuzz noise down the line, it’s very faint the sound.
Its probaby intermittent and you may have to listen when you get the disconnection.
What does your master phone socket look like?
Its quite common to get an intermittent bad connection between the front socket of the bottom section. and the test socket, which is why I suggested that you tried to prove it.
The problem is, that if the issue is being caused by your internal wiring, you would be charged for any visit.