Buy yourself a £5 corded phone, plug it into the test socket behind the removable faceplate of the master socket, dial 17070 and listen for any noise on the quiet line test.
Oh and to add, we do not have a phone. All phone tests by engineer have been fine.
We and the engineer only found 2 sockets, one could not be the master socket and he deaded that one. Replaced the other socket and problem still occurs
So as far as you know, you now only have one working phone socket? The engineer gave you a new master socket. Do you now have a master phone socket with a plug for the phone and a plug for internet on the same socket? No microfilter needed?
As @licquorice says you really need a corded phone so you can check the line from the test socket which would be located behind the master socket face plate, you could also then check the extensions to make sure they are disconnected.
It sounds like you have found the master socket as there should only be one hole for a phone to plug into behind the master socket face plate. Pluging a phone into that test socket eliminates all internal wiring whist you do a quiet line test.
A new master socket is not necessary I'm in a 1950s property and I would think my master socket is 30 years old judging by the really old bt logo on it, I need to use a microfilter. I was just trying to help you locate where you're master socket is.
Try leaving the router plugged into the test socket with a new filter, if you still get drop outs you will need an engineer visit to fix the intermittent line fault.