I am not sure how accurate this is, but a figure that was talked about alot while I worked at BT was an average of 35 connections between the exchange and the end user. That is a hell of a lot of connections to go wrong!

Most of these in the UK network will be non-IDC twisted or crimped connections. I always found it amazing how a DSL fault can be cured by simple replacing what looks to be a perfect connection by eye. There are 2 tests we used to do when looking for bad lines, the first indicator is one anyone can do, can you hear the DSL hiss or tones over the phone even with the filter on? If yes line = buggered.

The other test involved looking of what we called "The Mole" I have no idea what the technical name was. Once you had done it enough times you could see the fault on the scope as the shape was often similar, then it was all running about trying different connection points. Now you'd be amazed how many guys at BT call that advanced diagnosis!

Cheers

Cris.