Using a SIM PIN

Put a PIN in your iPhone SIM.

If you do not and the phone gets stolen, thieves can change the iPhone SIM and put it in another unlocked phone. And with the SIM in another phone they can request an SMS code for resetting the password to all your accounts, including iCloud, Facebook, Instagram, Twitter, etc. Even with two-factor authentication enabled, they can use this method to get the code and access your accounts.