If you're using a cell phone that operates on a GSM network (such as AT&T Wireless or T-Mobile), your subscriber information is stored on a SIM card. You can secure your SIM card to protect it if it is lost or stolen by setting a unique PIN code. If you've inadvertently locked your SIM card and are unsure of the code, there are a few steps you can take to reset the PIN and unlock your card. Carriers set a default PIN, and your card also has a unique PUK (PIN Unlock Key.)
Type in the default PIN code as set by your carrier. (AT&T Wireless uses "1111"; T-Mobile uses "1234.") Press "OK." If you receive an "Incorrect Code" message, proceed to the next step.
Re-enter the default PIN code until your phone displays "Enter PUK Code." (Three incorrect PIN entries will require the use of the PUK code.)
Contact your wireless provider's customer service department by calling the toll-free number. (This is generally listed on your invoice.) Request the PUK code for your phone--you will need to verify your account before the code is released to you.
Type in the eight-digit PUK code and press "OK." You will be prompted to enter in a new PIN code.
Type in a new four- to eight-digit PIN code of your choosing, then press "OK." Re-enter the new PIN code to confirm. Press "OK." Your phone is now unlocked.
There is no way to recover a forgotten PIN code, as carriers do not have access to a code once it has been changed on your device. Resetting a forgotten PIN is your only option.
If you receive an error message at the end of step five, you have entered one of the codes incorrectly. Re-enter the PUK code carefully and try again--10 incorrect entries will result in a permanently locked SIM card. Re-verify the PUK code with your provider if necessary.