Sounds pretty much like the extention cable is at fault. When I first set up my BB with the then Freeserve, now Wanadoo I could get no signal whatsoever to the modem on a 25 foot extention lead. I had a dedicated socket fitted by BT and a 30 foot trailing lead from there to the modem and all is fine.
Their advice is sound, an RJ11 extention cable should sort the problem out for you. Just get one long enough. Don't for get your filters.
If you don't mind paying a little extra, BT can set everything up for you with a dedicated ADSL Socket and lead to your PC. They will also test everything to make sure it all works before they leave.
Btw, with a dedicated socket your other filters will become redundant.
The normal extention leads are fine for dialup but not good enough for ADSL.
Hope this helps.