Some time ago when I was with Orange broadband >:( >:(during one of my many helpline enquiries about loss of service, they booked a BT engineer to check the line, he phoned one day to say there were no faults to be found, I pressed him further as to what could be my problem and could I do a line check myself, his reply surprised me, he said if I pick up the phone and hear a clear dial tone with no crackling or other interference the line was OK for broadband, in fact it was possible to get broadband down a wet piece of string. I would welcome comments from those more technically minded than myself as to the validity of his statement :o
The more scientific approach is to dial 17070, then select the quiet line test, 2, but basically what he said was correct. It's even possible to have ADSL working on a line when the voice service is not, and there is no dial tone.
That said, the quality of line obviously impacts on the ADSL service, usually in the form of restricting speed.
Thanks Rik, I really thought this guy was fobbing me off.