Why is my phone only saying no service

Optimize the range of the mobile network, see 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

Normally because it has no service. In other words, it cannot connect with tour service provider to get cell / mobile service.

See here: link-> If you see No Service or Searching on your iPhone or iPad - Apple Support

and contact your service provider.