The app does not control the Mavic's flight. So nothing happens, other then you can't see the video feed or use the controls within the app. You let the call go to voicemail and switch back to the active app.
I can understand you being paranoid using your phone because getting phone calls is a distraction. Also, the notification can take up a good portion of your screen. If you were flying outside LOS, then it could be a problem until the phone stops ringing. As @Drone Master said, airplane mode is your friend.