Hi Gwenillian. Welcome to justanswer.
Have you bought a new sim of BT one?
You need to activate your sim. Have you activated your sim yet?
Are you receiving any signal on your phone when you put BT one sim?
Did you get a confirmation message on your phone post activating the sim card?
Activate your sim once again by clicking here. CLICK HERE TO ACTIVATE
When you search manually for available networks, do you see BT network?
What is your area postal code?