IPhone 13 iOS 15.3.1 corrupted

I’ve downloaded 15.3.1 but iPhone13 won’t install it.

the first attempt got stuck on verifying update so I deleted it and downloaded a fresh copy

tried a few times.

Now it keeps checking for updates and says “Unable to Check for Updates”

It cant call out or receive calls.

erase all data gave an error message saying the phone wouldn’t work afterward

even tried a full reset and it wouldn’t let me

any hints to go back to 15.1.1? Or fix the update?