Also for AP not sure if you are using autonomous or lightweight controller.
If lightweight controller and pre-22.214.171.124, was an issue where 7920 connection to CallManager could get reset as the packets could get aged out within a 1 second period.
The workaround was to set the 802.1p tag for platinum to 7 vs 6. Would suggest to upgrade if possible though.
Also the 7920 sleeps every 2 seconds and doesn't use DTIM to check for queued packets, so could potentially be up to a 2 second delay. Would need to ensure proxy ARP is enabled on the AP with the 7920.