I understand "Calls from iPhone" is gone. That has been known to happen, and it's also been known to spontaneously reappear on its own without any overt action on your part. Rather than wait an indeterminate amount of time, try "signing out" of that Apple ID Account in FaceTime's Preferences, followed by signing in again. Then, wait a little longer.
Upgrading to macOS Big Sur involves an extensive amount of security-related communications involving all products and services using your Apple ID. That is certain to take a long time. Hours perhaps, but not days.
You are not addressing Apple on this website.