Some first-time students are really shy. They may cover the camera and not say anything or they may point the camera at the ceiling. You should patiently try to engage them in conversation. Start introducing yourself and telling them a bit about yourself. Ask them a few simple questions. Usually shy students will get over it and begin talking to you.
If after a few minutes, there is no improvement, you have the option to very politely say goodbye and hang up. For example: "I'd love to help you, but you should come back when you are ready to practice." Then smile and wave goodbye.