According to the website https://www.bt.com/help/mobile/manage-service/how-do-i-unlock-my-mobile-phone-or-tablet- , the following models are locked.
Samsung Galaxy S10, S10 Plus, S10e, S9, S9 Plus, the A40, A8, A6, J6, J6 Plus J3 (2017) and J4 Plus
Huawei P Smart, P20 Lite, P20 Pro and Huawei Mate 20 Pro, Huawei P30 and Huawei P30 Lite
iPhone XS, XS Max and XR
Motorola G7 Power and Moto g6 Play
And as I have found out today the Google pixel 4a ..... Not on the list but locked only bought as it has an esim function and I need to use another providers card for work save me having two phones..... Still waiting for a reply from BT as to why..