Say that this person has left a call the others are still in.

Says that the person making the request has walked out of a call. Use it whenever somebody leaves, not only when the call is over: it is what keeps the number of people shown beside a call honest, since that number would otherwise count everybody who had ever been in it. It never ends the call and never affects anybody else, so it is safe to send more than once, and it is not refused for being over your allowance. To end a call for everyone, use end_call_session.