For iOS devices, tap Settings>Phone>Select SIM PIN. Switch SIM PIN ON and enter the default SIM PIN. Tap "Change PIN," and enter the default SIM PIN. Enter your new 4-digit PIN and re-enter your new 4-digit PIN. For Android devices, tap Settings>Security>Set up SIM card lock and then select "Lock SIM card." Enter the default SIM PIN and select "Change SIM PIN." Enter the default SIM PIN and your new 4-digit PIN. Finally, re-enter your new 4-digit PIN when prompted.
You can check the device manual for other operating systems since the PIN setting process varies from device to device. When you have already set up a PIN number, your device will occasionally require you to enter your SIM PIN, so you should always remember it. The default SIM PIN varies, so if you do not know yours, you can contact your mobile operator for a correct one.
Your device allows you to enter the PIN number three times. If you exceed this number, your mobile operator will block the SIM, and you will be restricted from using the network. To unblock your SIM, you will need a PIN Unblocking Key (PUK) code. If you enter the incorrect PUK code, many times your SIM card will permanently block. However, you can get a new SIM card by contacting your mobile operator.