I work in a place where we pulled a bunch of iOS 9 devices out of retirement for FaceTime calls and we are also experiencing this issue. We have found a work-around.
On the newer (iOS 14) device, go into Settings > FaceTime and allow your Apple ID to be used for FaceTime calls. Touch to place a checkmark on your Apple ID under "YOU CAN BE REACHED BY FACETIME AT".
As long as the email is checked and active, you can make/receive FaceTime calls between the older and newer devices. You may need to have the older device call using the email address rather than the phone number, we have not gotten the number to work consistently by enabling the Apple ID for FaceTime calls. The email address always works, however.
This is a cumbersome fix on the scale we are using these, hopefully it's just a bug in the newest iOS release and Apple is working to fix.