worth checking out the phone wiring in your gaff n'all. That's the first foboff the helpdesk'll give ya.
If it's possible to move your kit about, plug yer router direct into the master phone socket & disconnect everything else. Problem goes away then it's likely the phone wiring in yer flat, in which case it's easy sorted.
Problem still occurs you have a pretty much solid case to go to BT or Sky.
if you want more deets on sorting your house phone cabling out drop us a PM