If you are experiencing difficulties making Calls from iPad via your linked iPhone or WiFi Calling, it may be necessary to first fully disable both Wi-Fi Calling and Calls on Other Devices on the linked iPhone.
Settings > Mobile Data >
- Wi-Fi Calling - Add Wi-Fi Calling For Other Devices - set to OFF
- Wi-Fi Calling on This iPhone - set to OFF
- Calls on Other Devices > Allow Calls on Other Devices - set to OFF
Temporarily disabling FaceTime on both iPad and iPhone is also recommended:
Settings > FaceTime > FaceTime - set to OFF
Next, force restart your iPhone and iPad:
Force restart iPhone - Apple Support
Force restart iPad - Apple Support
After both iPhone and iPad have been restarted, re-enable FaceTime on both devices - allowing a few minutes for the service to fully reactivate on both devices. Now re-enable Calls on Other Devices on your iPhone - then re-enabling Wi-Fi Calling on iPhone. It may take many minutes for your carrier to re-enable Wi-Fi calling services; be patient.
Finally, on iPad, re-enable Calls from iPhone:
FaceTime > Calls from iPhone > Calls from iPhone - set to ON