We have a Mitel 5000 system and recently purchased SIP 6900 series phones. However, in dealing with our vendor, it seems like we have to purchase two licenses, one for the Mitel phone system (Category F licenses) and another for Phone Manager. Is all of this really required? I currently have phones that show up with features like DND listed as "unlicensed", but it seems silly to me that I have to buy a Phone Manager license for do not disturb to work???