Check your account settings in outlook (or outlook express). Go to Tools - Accounts - Properties and click on the connection tab. If you're using Outlook then you want to select "connect using 3rd party dialler", if outlook express it'll be something like "connect via any available connection". I am in the same position as yourself and use BT most of the time, but never have any problems downloading freeserve mail through BT.
As ex-sim guy says though, remember for sending you can only send freeserve to freeserve if you are connected via BT (but obviously BT to any) and vice versa.
Drop me a line if you still have no joy - it's not a huge problem to sort.
CM