Camp-on to a busy or idle extension. If the extension was busy, the user will be alerted as soon as it goes idle and a call will be automatically placed. If the extension was idle because the person was not at their desk, the user will be alerted as soon as the extension is used again. Equally an external call coming in on a DDI line finds the extension or DDI busy. If the caller is registered in the Company Directory it cab call him back, or if the user is not registered, then use a function number to register the callers number for call back.