Should I set a SIM PIN on my iPhone 16, or is it already enabled by default?

If you are using an eSIM you do not need a SIM PIN. It is more for removable SIMs, where if your phone is stolen the thief can remove the SIM and use it in another phone.

SIM PIN setting is optional.

If you forgot or don't know your SIM PIN

Never try to guess a SIM PIN or PUK code.* The wrong guess can permanently lock your SIM card, and then you would need a new SIM card. If your device asks for a PIN or code that you don't know, here's what to do:

1. Contact the carrier that gave you the SIM card. If you aren't sure which carrier to call, remove the SIM card and check the card for the carrier's name or logo.
2. 
3. Ask your carrier to help you unlock your SIM card using the default SIM PIN or PUK code. 
4. If you can't unlock your SIM card using the SIM PIN or PUK code or if an alert says, "PUK exhausted," ask for a new SIM card.

*If you enter the wrong SIM PIN too many times, your wireless carrier might be able to give you a Personal Unlocking Key (PUK).