Hi there WEVH,
Thanks for joining us in Apple Support Communities! We understand calls are dropping when switching the audio source from your iPhone to your iMac since upgrading to macOS Big Sur. You've come to the right place, and we're happy to help.
Start by making sure Handoff is set up correctly with the steps in this support article:
Use Handoff to continue tasks on your other devices
Afterwards, follow the guidance in this link to be sure you set up iPhone Cellular Calls:
Make and receive calls on your Mac, iPad, or iPod touch
If this behavior continues, you may find the additional information in this link helpful:
Allow phone calls on your iPad, iPod touch, and Mac
After checking all your settings with the guidance in the links above, test the issue again by trying to switch the audio from your iPhone to your iMac during a phone call, and let us know how it goes.
Cheers.