If you update or restart your iPhone or iPad, you might see one of these alerts:
Cellular Update Failed
If you see this alert, it might mean there's an issue on the cellular modem of your iPhone or iPad. Your iPhone can't make or receive calls or text messages, or access cellular data until you update your device.
You'll also see a warning sign and No Service instead of your carrier name in the status bar, and you'll see Error next to Settings > Cellular and Settings > Phone.
If you restore or erase your iPhone or iPad you won't be able to activate your device. If you try to activate your device, the activation will fail and you'll see a message that says "An update is required to activate your iPhone."
Apple Pay Not Available
If you see "Your iPad cannot use Apple Pay until it has been updated," you won't be able to use Apple Pay or Wallet for payments or access until you update the iOS on your device. Some Wallet passes might still be available while payments are unavailable.
If you restore or erase your iPhone or iPad, you might not be able to activate your device.
Contact Apple Support for more help.