To prevent unwanted use of your credit you can set a PIN code on your SIM card, by following the security options in your handset settings.
The default SIM card PIN is set to 0000, if you make a mistake a get it wrong three times in a row, don't worry: you can reset this using the PUK code printed on the holder which surrounded your SIM card.
If you are unfamiliar with the unlock process; we recommend that you call customer services from another phone on 0207 87 17 669
and we'll talk you through it.
Please be aware: If you make a mistake at this point you could cause permanent damage to your SIM card