Belkin do various long lengths of shielded RJ11-RJ11 cable which is pretty decent.
I use that ADSL Nation cable all the time now and have had very good results with it.
Of course, I know that if you could site the router nearer to the master socket then you would have done so. So I'm not helping you by saying that of course it would be better to have a long run of ethernet cable than a long run of phone cable. So I won't. :-) That's just how things work out some times.
But where I have to deal with difficult lines, and for any business customers, I tend nowadays to do BT Openreach "change-point-of-entry" as a matter of course, as it keeps things neat and tidy, and on long lines gives you that little extra boost in terms of performance and reliability as the master socket is then exactly where it is needed and cables are short. Change point of entry isn't cheap [£120+ ?] but if you're struggling it might make the difference. And often cleaned-up wiring and a new, clean master socket is worth a little bit too. In fact, I did booked one with BT for a customer last week.
If you're on a short line then it doesn't matter much, of course.
