A SIM pin can prevent anyone who gets unauthorized access to your iPhone from accessing the cellular data network. While there's tons of pre-emptive measures you can take, including using Touch ID, a password, or passcode to secure the Lock screen, a SIM pin is yet another layer of protection for anyone who needs it.
Once the SIM pin code is setup, your iPhone will ask for it anytime you swap cards or reboot.
- Launch the Settings app
- Scroll down and tap on Phone settings
- Scroll down to the very bottom and tap SIM PIN
- Switch the SIM PIN option to 'ON'
- Enter the default PIN set by your carrier (AT&T is 1111, if you're on a different carrier check with them, or Google it)
- Tap Change PIN.
- Enter the current (default) SIM PIN again
- Enter your new, custom 4-digit PIN code