When using the bt_test@startup_domain login you are instructing the RAS to route you directly to the bt.net intranet. This is probably the single most important thing you can do before calling your ISP when you have a problem. If you can connect using this login then the ISP should know that the problem is not at your end or in the local exchange. The problem must lie with either the BT RADIUS server or at the gateway/ISP RADIUS server. Unfortunately the login only grants you the right to access www.bt.net and nothing else.
The labels (1), (2) & (3) indicate the sequence of events.