It's somewhat user-unfriendly that your outside parties can't call into a scheduled conference a little early. Everyone want to be on time to a meeting, right? So they call in a few minutes early and 3cx promptly drops their call, causing them to panic, thinking something is wrong. No other conference service I've encountered won't allow you to call in early. Maybe a system-defined pre-scheduled-time grace period of 15 minutes or so?