CCO doc's state that when enabling voice vlans on cat switches any untagged traffic (native) is sent according to the default CoS priority of the port and the CoS value is trusted for 802.1p or 802.1q traffic.
The question is, since CoS resides in the 802.1q/p tag, how can it classify a CoS value to traffic that is not in fact tagged with 802.1q/p ??
They are not saying that the traffic is being classified, they are saying that because it can't be classified in this manner (because it is untagged), the traffic will be sent out at the default priority, i.e. in the default queue, for that particular port.
They are referring to a default COS value. which will be 0, unless it is changed.
You can, however, make this any value you would like, but it is global on the 4006. On a 35XX (and I think 65XX) you can set it on a per port basis, which is nice if you need to connect a non-trunked voice device like a Unity server or a low end phone. So if you set the default to 5, any inbound untagged frames that are then forwarded out on a trunk will have the COS set to 5.
This example shows how to set CoS equal to 7 in all unclassified frames received on the switch and verify the configuration:
Console> (enable) set qos defaultcos 7