Change the SIM PIN
The SIM card PIN code is meant to protect your data against unauthorised access. Each SIM you insert into a smartphone has a PIN code that you need to enter in order to activate the card. Some mobile providers use standard PIN codes, like 0000 or 1234, for all their SIM cards, while others generate unique PIN codes for each SIM. Whatever the case, it is safer to change the default SIM PIN code to one of your choosing.
If you do not and the phone gets stolen, thieves can change the 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.