I'm probably not running the latest version of the Windows 3CX Phone as I don't use it that often. (Much more often on Android). The thinking may be that the softphone client is probably used more often outside the "home" LAN, and if that is the case, you probably won't have any control over QOS anyway. Unless you local LAN is running close to it's maximum, then QOS will only be of concern on traffic leaving and moving to the WAN (your VoIP trunking to/from the PBX), and QOS, in that case is only effective if honoured by your internet provider, certainly more likely if your VoIP provider and ISP are one in the same.
Are you experiencing any degradation when using the 3CX phone without QOS?