Before using your Microphone for Voice Calls, and your Camera for Video Calls, you may have to grant permission for apps to access your Microphone and Camera. Follow this guide to quickly get access like a VIP in some fancy nightclub.
To get started, go into your System Settings -> Privacy -> Microphone AND Camera.
You'll see two toggles under the "Allow apps to access your camera/microphone," turn both of these to "On" and voilà!
You've just enabled permission for apps to use both! Do a little happy dance, you've earned it!