If your BT Mobile SIM won't work first time, your device could be locked to your previous provider. Learn how to get it unlocked.
Here are some answers to help you
How do I unlock my mobile phone or tablet?
Help unlocking your model of phone
Unless you're using your BT Mobile SIM with a brand new phone, it might be locked to the network of your previous mobile provider.
Go back to: What can we help with?