Have you got a filter on all lines in the house, people usually forget about sky boxes

Alot of the clients I do work for think that the filter in the box they get from the ISP goes on the dsl line, rather than the voice lines.

On the other hand, if its a 'BT' problem you will need to go through your ISP and make a complaint.

BT - as in the residential phone line provider, will not guarntee data transmission over the standard phone line. Your ISP will have to log a fault with BT wholesale on your behalf who will test the line/equipment. You cannot talk to BT wholesale yourself. If it does get to this stage and BT say they cant find a fault then it will be down to you to prove where the fault is - big headache.