BT supply the phone line in order to make and receive voice phone calls. If you can do this then that is all thats required of them and you will get nowhere. Even if your ISP is BT you will find that they have a minimum limit for BB speed (something really low like 150kbs) and wont do anything unless it is below their limit. The really frustrating thing is that there is not much you can do about it except keep complaining and quote your noise/attenuation figures at them.
If you get an engineer out it will cost you a lot if no fault is found.