If you miss a call from a student who specially requests you, we automatically set you to invisible. To the student, this will look like you've just signed offline. This is to prevent students from calling you repeatedly while you are away from your computer and becoming frustrated. Once you return to you computer, you can set yourself as visible again by using the dropdown menu on the Dashboard tab of your tutor account.
If you're removed from a Priority Hour, we'll also set your status to "invisible" automatically for the same reasons.