Yep thanks @SammyM too.
I know it sounds threating and scaremonging about a possible charge. All you need to do is to make sure there is nothing after the master socket that 'could' (I stress could I am by no means saying it is) be something like a dodgy phone, extension cable or something else, like a dodgy filter.
The wonderful world of technology, it'll work at some point then decides it doesn't want to anymore and it may not be obvious why.
So I would disconnect everything, just plug your router into the master socket, and even test socket if possible. See if there is any noise on the landline, so try either of those - 17070 option 2 quiet line line test. It has happened to my landline and to be honest if I didn't read posts on here I wouldn't have a clue what to do. Engineer came round and found the problem was in the drop wire outside (no charge).
I had a similar one, when I was moving out of my 2nd year house from Uni. Typical, whilst moving stuff all the electric went out. Great when you're trying to do something - "Oooh a power cut I thought". I walked outside and realised houses next door still had power (a bit of tip is when you can still see the lights are on).
Back in the day (cor I feel like Uncle Albert from Only Fools now). So 'during the war' I think I had just gotten one of my first mobiles. A brick. Rung energy company up and they said "If it is beyond the meter we'll charge, if it isn't we won't". Turned out the big fuse holder before the meter had disengrated, so no charge and not our fault. Fun and games trying to move out in the dark.
So a similar thing really, you just need to be certain you have done all the tests you can and be certain as much as possible it is not beyond the test and master socket.
