O2, a UK -based communications company, offers mobile phones and SIM cards. Whether you're using your own phone with an O2 SIM, or one of O2's devices, you can protect your SIM from unauthorised use by setting a PIN (Personal Identification Number) code. When active, you will need to enter the PIN before your phone can be used. O2 uses "0000" as the default code. Three incorrect entries of a PIN code will cause your SIM to be blocked.
Power the phone on. If your SIM card is barred, an error message will display. The error message will probably say something like "SIM locked", "enter PUK code," or "blocked," depending on the phone manufacturer.
Click the link in the Resources section to be taken to O2's "Unblock My Mobile" page.
Enter your mobile number in the "Enter Your Mobile Number" field, toward the bottom of the Web page. Click "Go."
Follow the instructions to obtain the PUK (PIN Unlock Code) for your device.
Press your phone's "OK" button to enter the PUK code. Type in the code and press "OK." Follow the instructions on your phone's screen to reset your PIN code and unlock the device.
Ten incorrect PUK code entries will result in a permanently blocked SIM card. You will need to obtain a replacement SIM card from O2.