From a Mitel prespective? The phones can be set to tag their packets with layer 2 and layer 3 priority. You then need to setup you Cisco gear to work with those values on the network. DSCP default is 46 for the Mitel. Believe that is the same for voice on Cisco. Therefore any routing between VLANS need to expedite packets marked with this value. Most systems I have setup I have setup with the phones tagging their packets with layer 2 priority of 6. Therefore the layer 2 switches need to be setup to respect that value.
