If you're using a Mac to join your Call.Group calls you may need to check your system preferences to make sure that you are allowing Call.Group and your Internet browser to access your camera, microphone and screen.
How to check your Mac permissions
Click on the Apple icon at the top left of your screen and navigate to 'System preferences'.
Click on 'Security & Privacy'
You'll see a list of different elements on the left and a list of apps which have access to them on the right. Make sure that for your microphone, camera and 'screen recording' you have allowed access to 'Call.Group' as well as your Internet browser.