Error 'iPhone unavailable' on my iPhone 12

You either gave the wrong passcode or you put it in your pocket, purse, etc. and it accidentally dialed while rubbing against your pants. This did not happen all on its own.

If you forgot the passcode on your iPhone, or your iPhone is disabled or unavailable -

Click on the link for instructions on how to reset your passcode without a computer if you have iOS 15.2 or higher.

If you see a iPhone, iPad, or Apple Vision Pro unavailable message or Security Lockout screen - Apple Support

Click on the link for instructions on how to reset your passcode using a computer.

If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

You must have given a wrong iPhone passcode some way, but your iPhone most definitely knows the correct passcode. It is stored in a secure place called the Secure Enclave where the passcode cannot be modified or bypassed otherwise. As stated, if you are certain you are entering the correct passcode, it may have come in contact with your body or something else sensitive in which it could have entered incorrectly. The best way to proceed would be to reset your passcode.

If you forgot your iPhone passcode or your iPhone is disabled - Apple Support