How to use phone number with I message

Does your new carrier have international texting enabled? That’s required to change any iMessage or FaceTime settings.

There’s further information here→Add or remove your phone number in Messages or FaceTime - Apple Support

Specifically:

If your phone number is unchecked and you can't select it, or you have some other issue, follow these steps:

On your iPhone, iPad, and iPod touch:

• • Go to Settings > Messages > Send & Receive. Tap your Apple ID, then tap Sign Out.
  • Go to Settings > FaceTime. Tap your Apple ID, then tap Sign Out.

On your iPhone:

• • Go back to Settings > Messages and make sure that iMessage is turned on.
  • Tap Send & Receive, then choose “Use your Apple ID for iMessage” and sign in with your Apple ID.

Using the same Apple ID, sign back in to Messages and FaceTime on your other devices. 

If you still need help, change your Apple ID password and repeat the setup steps. If that doesn't work, contact Apple Support.

Did you change phone numbers also? If you did go to Settings/Messages, tap on Send & Receive, then your Apple ID and log out. Next go to Settings/Phone and correct your phone number if it is wrong. Finally, go to Settings/Messages and log back in, and make sure your phone number is correct and checked.

Then still sign out and sign back in to iMessage.