If you bought your mobile phone directly from a mobile provider, it might be locked so it'll only work with one of their SIM cards.
To find out if your phone is locked, put your BT Mobile SIM into the phone and then turn it off and back on again. If the SIM doesn't work then it's probably locked.
You might also get a pop-up message saying "Handset locked" or "SIM not valid".
To get your phone unlocked, just make sure you've got:
You'll then need to contact your previous mobile provider. You'll either get an unlocking code to put into your phone or your previous provider will unlock it for you.
With most phones this should work straightaway. With some iPhones it may involve connecting your phone to iTunes and putting in your Apple ID. (Make sure you have the latest version of iTunes and iOS before you do this.)
If your phone's from an old contract or an old mobile provider, you might not be able to unlock it online. It may be best to go to a mobile phone shop.
No, unlocking is completely legal. But if a phone has been reported stolen or missing, it will have been blacklisted (or blocked). Un-blacklisting or unblocking a phone that wasn't blacklisted by you is definitely illegal. That's why we only recommend going back to your previous mobile provider to unlock your phone.