This issue occurs because the phone number can become unselected in the iMessage settings during the eSIM setup process.
To resolve this issue:
Open the Settings app on your iPhone.
Navigate to Messages.
Select Send & Receive.
Ensure that your phone number is checked under the "You can receive iMessages to and reply from" section. If it is not selected, tap on your phone number to re-enable it.
This should restore iMessage functionality.
Further Assistance: For additional troubleshooting or detailed discussion, visit the Apple Support Forum thread on this issue: Apple Discussions Forum.
If the problem persists, contact Apple Support for further guidance.