Hello John,
Welcome to the Apple Support Communities! We understand you've experienced an issue where calls are dropped when you switch from your iPhone to your iMac, since you've upgraded it to Big Sur.
If this is still occurring, begin by restarting both devices:
Restart your iPhone
Log out, sleep, wake, restart, or shut down your Mac
If the issue persists after restarting, check to see that both are running the latest versions of the operating system.
Once you have completed those steps, try switching audio sources and see if the issue persists. If it does persist try switching to another audio source, like headphones, and let us know if there is any change.
Cheers!