Usually broadband dongles are customised for their parents' networks - in your case, BT. Since you are not a BT customer, then of course it won't register your non-BT Broadband number. You may also find that the original BT SIM card was customised. It might be possible to download new cracked firmware from somewhere, but you will have to search for it, and it may not work even then... Sorry. The probable solutions are either to go with BT, or abandon this route?