I have had considerable problems with the telephone line and my broadband connection, the line was crackling and prevented practically any conversation and broadband would not connect 99% of the time and when it did it disconnected immediately. I called the telephone company who sent an engineer out to check the line, he found the the main box inside the house had damp round the connections which was only detected by his meter. He replaced the box and the phone worked fine but the broadband did not, I was about to telephone the ISP when I decided to disconnect the external USB modem and reconnect it. Instant connection which is still on over three hours later. It seems the modem had "memorised" the settings whilst the line was bad.
Now all you experts out there will no doubt have sussed this out but if there are any other beginners/intermediates with this problem it may be worth trying these two solutions.
Good news he replaced the box with a telephone plug (resists damp better) so I can now plug in an extra phone upstairs, bad news, the box being in the house I have to pay for his time.
of how an irritating problem can be caused by something extremely simple, and yet potentially very difficult to diagnose. Many of us have experienced line crackle at times, and it took me a month or so to figure out that when it happendd to me a couple of years ago it was being caused by a pavement line box getting flooded each time it rained. BT sorted it all out very rapidly, and I haven't had a repetition.