If a CoS Value comes into the AP, that takes priority over all other QoS policies.
Then the dot11 phone setting is used to "identify" a phone and provide priority.
Then QoS policies are looked at.
For QBSS Draft 6 supported by the 7920, QoS Element for Wireless Phones to enable (no dot11e)
For QBSS Draft 12 IEE802.11e supported by the 7921 also tick dot11e
dot11 phone dot11e
Enable WMM for the Vendor specific que values to be offered in the beacons (per radio) does not hurt to leave on in most cases unless they ask you to remove it
Enable Low latency where needed so that packets are dropped after certain level of discard. Not for 7921 due to its dynamic queing mechanism.
When optimising for 7921 phones, design guides state to match RTP and SCCP by DSCP and map to relevent cos (possibly not needed but hey!)
class-map match-all _class_RTP0
match ip dscp ef
class-map match-all _class_RTP1
match ip dscp cs3
set cos 6
set cos 4
service-policy input RTP
service-policy output RTP
Also when optimising. Select Optimized for voice, but remove low latency queing from Stream as 7921 dont need it!