Do the checks, and then......
Once you've done all the usual checks, as suggested by Jim., try calling BT more than once.
I've had very different experiences from calling them. Sometimes they don't know/care, but sometimes I've had really helpful individuals on the line, going well beyond that "the line is OK sir" response.
They do have a script to run to, but with some of them they'll go well beyond that. The number i've used is 0800 111 4567.
Once they just could not be bothered. A letter to the CEO resulted in a quick call from their problem resolution team, visit from an engineer, and admittance that it was their problem...........but I guess that many of the problems they deal with are with customer's kit/set up.........most of mine are!!