Let's start from scratch.
You are using ADSL ... so each phone point requires a filter .... the filter should have 2 sockets ... an oblong one with 6 connectors (phone) .... and a square(ish?) one with just 2 connectors (RJ11 - modem)
to be certain - wherever the line comes into the house there should be a special socket (nte5) which is "split" into two halves .... the top (internal) part can be unscrewed and unplugged from the fixed back (external) - and so you'll be left with a socket connected directly to the feed - see
http://www.amazon.co.uk/Solwise-PBX-NTE5/dp/B0 00LQ9UNQ/ref=pd_bbs_sr_1/026-7659121-6315630?i e=UTF8&s=electronics&qid=1179409864&sr=8-1
from here you can isolate the internal and external wiring.
plug a filter+ modem in here ... and you are certain nothing else can interfere
The phone and dsl should work without filters .... but not at the same time (that's what the filter does).
you've disconnected all the phones .... but have you plugged a phone into the socket you are using for the modem?
have you moved the modem to one of the other sockets?
see how you go