Message centra number
How can I chake my message centre number iPhone 7 iOS 14.8
iPhone 7, iOS 14
Message center number depends on your carrier, you can't just change it. Contact your carrier.
See also this Apple article: If you see No Service or Searching on your iPhone or iPad
If you see No Service or Searching on your iPhone or iPad - Apple Support
If you still don't have a 'service' after that, see -> iPhone 7 Repair Program for "No Service" Issues
iPhone 7 Repair Program for "No Service" Issues - Apple Support