First verify that Settings > Sounds > Ringtone is set to a sound of your choice - and that the Ringtone volume (at the top of this settings page) is set near the maximum.
Completely log-out from FaceTime on your iPad - restart - then login again to FaceTime.
If that doesn’t resolve the issue - and you are sure that both your iPhone and iPad are correctly enabled for Calling on Other Devices - then instead log-out of iCloud and other core Apple services on your iPad - restart - then login again.
Let us know how you get on.
Hope this helps to resolve your problem.