Your SIM card came with a default PIN from your carrier. You can use it to set up an optional SIM card lock, which prevents others from using your cellular plan and accessing account information if your phone is stolen.
If you add a SIM lock, you will be prompted to enter the PIN each time you swap SIM cards or restart the phone.
When prompted, enter the PIN used to set up the SIM lock.
If you enter an incorrect PIN multiple times, the SIM will be disabled.
Contact your carrier for a PIN unlock key (PUK) to re-enable the SIM.