but I assume that it's approved for use with the BT ADSL service, not all broadband modems are.
It sounds as if your machine is having a problem validating itself with the BT Home Gateway, and the answer may be in the settings for the dial up connection.
go into your dial up network connections and right click on the ADSL connection icon. Select 'properties' from the menu and then click on the 'Networking' tab.
make sure that the server type is set for: PPP:Windows95/98/NT4/2000/internet and that there's a tick against the following items:-
QoS packet scheduler
File and printer sharing
Client for Microsoft networks
There are some more advanced settings you could check, but try these first.