SIM cards hold important subscriber information, and your GSM phone won't operate without one. Most phones allow you to set a security PIN to protect your card from unauthorised use--a handy feature until you're unable to remember the code you used. You can try the default password, but continued wrong entries will result in a blocked SIM card. In this case, the password you'll need to unlock the SIM is called a PUK, or PIN unlock key.
Enter in the default PIN code if your device is requesting a PIN. For most phones, this is "1111." Press "OK." If the device does not accept the code, continue to enter it incorrectly until you are prompted to enter a PUK code.
Contact your provider's customer service department by calling the toll-free hot line. If you don't know the number, it should be listed on your invoice.
Request the PUK code for your device. You will need to verify your account details--phone number, account security password and account holder's name--before the representative will release the code. Write down the code.
Use your phone's keypad to type in the eight digit PUK code. Press "OK."
Enter in a new PIN code of your choice. Press "OK." Re-enter the PIN when prompted to confirm it, then press the "OK" button. Your SIM is now unlocked.