If you set in
advanced > Conntrack/Netfilter > Tracking/Netfilters > SIP
then the use of traffic to categories in
QOS > Classification
is unreliable. I found that some traffic would be in the category "Unclassified" even though a rule would clearly map to it. When the SIP tracking helper was disabled, the traffic was correctly set to the appropriate classification.
The traffic in question was SIP traffic from my Asterisk (Trixbox) PBX.
I am using a Netgear 3500L with the USB/VPN Beta 16 load.