If the guest cannot allow access to his camera and / or microphone, he will see the message 'we cannot see or hear you'.
Then, as an advisor, choose the 'audio by phone' option. As soon as you both have dialed in, the audio will go through the telephone and only the camera stream or the shared screen will be shared with the guest.
If you still have questions or you can't figure it out? Then contact email@example.com or via chat!