A SIM card is a “subscriber identity module.” This card contains a chip that holds your name and the telephone number in use on the phone. A phone can't make calls without a SIM card and if your iPhone’s SIM is locked, it is effectively shut off from the telephone, and so no communication can take place. You need to unlock the SIM card before you can make any calls.
The iPhone will give you an error message on the screen of the phone if it is barred from making calls. The phone has to access the details on the SIM card every time you want to make a call, and so if the SIM is locked it cannot complete your actions. Instead, it will inform you of the problem with the message “SIM locked.”
Types of lock
The term “SIM lock” refers to two different types of block. The iPhone displays the “SIM locked” message for when the SIM is password protected. This form of lock is not usually automatic. You, or a friend, must have enabled SIM locking as a security measure. The other type of lock is a restriction on the phone which only allows it to work in certain countries or with a specific network. You may see gadget shops offering an unlocking service, which is also known as “jailbreaking.” However, this service provides a work around for the restriction type of SIM lock. It will not help you with your password-protected problem.
The message that tells you the lock is accompanied by two buttons, “OK” and “Unlock.” This tells you what you need to do to end the lock. Pressing “OK” will not change the situation. If you press “Unlock” the phone will ask you for your PIN number. You would have chosen your own PIN number when you set up the lock. However, if you do not remember setting up the lock, then your likely source for the PIN number is on the card from which your SIM card was removed when you bought it. The PIN number usually accompanies the new SIM card.
If you enter the PIN number incorrectly three times, the phone will lock more securely and you will have to phone your network carrier to get a PUK number. “PUK” stands for “Personal Unlocking Key.” Your phone is now “PUK locked.” You have to get the PUK code right. If you enter it incorrectly, the SIM will lock permanently and you will have to replace it with a new one. However, you have 10 chances to enter the PUK before the SIM locks completely.
The PUK lock message has the same format as the SIM lock message. It has an “OK” button and an “Unlock” button. Tap the “Unlock” button and enter the PUK in the field that appears. Tap the “OK” button next to the field. Next you will be prompted to enter a four-digit pin. You can enter any four numbers you like, but remember them, because this will be your new PIN to unlock the SIM in the future. Press the “OK” button next to the PIN. You will be prompted to enter the PIN again. Enter the same number again and press the OK button next to the field to complete the process.