I recently purchased the IP500 with 5410 and 5420 phones. I was told that there is a way to setup the phone so that if one extension calls another and the call recipiet is on another call already, the display of the caller will let them know that the party they are trying to call is already on a call. this would then prompt them to either hang up, or set the ringback free using the button I've programmed. I cannot figure out how to get the notification to work. Right now they would call and get the user's voicemail after 4 rings which means they'd have to hang up, and call back to set the ringback free. this doesn't makea lot of sense. Any suggestions or instructions?