I haven't set it up on my router, I'm running the last version. You can either set the priority by MAC address or, probably more useful in the case of a PBX, give priority to one of the four LAN ports. Plug a switch into that port and run the PC with 3Cx and all extensions on that switch. That also keeps all internal SIP traffic away from the rest of the network. Bear in mind, that QOS will only have a affect on uploaded packets, giving priority to those you are sending out to the WAN. It won't help if others on the network are downloading huge files.