Phones are locked
Welcome to the Community.
I can't remember where I read this but I will try and find the BT Help Page that the info came from.
BT Mobile phones are locked to BT Mobile but can be unlocked by BT once you are out of the first 12 months of your contract.
phone is locked for first 6 months not 12 https://www.bt.com/help/mobile/manage-service/how-do-i-unlock-my-mobile-phone-or-tablet-
obviously there are other methods to unlock the phone for a small fee
I had used the page not that long ago for someone else