I used to have Plusnet with their supplied 'Technicolour' router. One day, a friend mentioned he had been trying to phone and got 'unobtainable'. I checked our line and sure enough it was dead, however I was still getting broadband, so I checked the strength and found it was down to just under 2 mb/s.
How long the phone was out for I don't know but from checking it to it being restored was 5 days.
I switched to BT after this.