You need to allow access to your camera and microphone before making a video call. Here are step-by-step instructions:
Android: Settings > Apps > App Name (Getcontact) > Permissions
iOS: Settings > Apps > App Name (Getcontact)
Here you can check and manage your camera and microphone permissions.
Also, when you tap on video call button, your camera and microphone permissions are checked. If you have not granted access, you will be forwarded to relevant screen.