we had a similar problem, despite many (and I mean many) phone calls to BT (our ISP) eventually after many changes 3 routers, 1 line change and lift and shift at the exchange we still had the problem, it was intermittant and sometimes it connected, sometimes it didnt (despite saying connected)
eventually it was solved (well we think!) by BT changing the filters at their end, contact your ISP and they can deal with this for you.
its a very frustrating problem and has taken 2 months just to get sorted, you have my sympathies!