I'm afraid that your ISP is giving you the runaround. Unless there is audible noise on the telephone, BT Retail will do nothing. They will only take action over voice problems. If the problem which you have only relates to ADSL, then it's your ISP's job to sort it out.
I'd not agree. We're not giving jamie the 'run around'. BT (retail) will also look at many other faults, not just ones that relate to audible noise. Many faults related to the PSTN service do not generate noise but can still be service affecting and are still investigated and resolved by BTOR on behalf of BTR.
Like Wholesale when dealing with broadband faults, their 'fix' is often to apply X or Y profile to gain stability. This is often not what the customer likes (i.e major speed decrease) but will make the line stable. C&W also have a fix for this issue.. but the customer may suffer lower sync on both the down and upstream paths.
BT *will* often investigate an incoming call issue directly because it is an incoming call on the PSTN (voice) side that is causing issues for other services on the same line (i.e broadband).
As we advised via a support ticket, as we have the option of going to BT first, it is good to take that opportunity and then if need be, we can raise with C&W to apply a fix (although might not be really dealing with the root cause - just masking it).
As BT have acknowledged the fault, it shows that there was one on this line and it is covered within their remit. If not, they would have likely kicked it back to the customer to raise with their ISP... as BT seem to do far too often.
Matt