This is in idle primarily?
Are shared lines used?
Does this occur when roaming? or even if not associating to a new AP?
Will need to investigate why SCCP packets are not getting exhcnaged properly.
Is this in certain areas?
Could try enabling a lower data rate, which could potentially be more reliable transmission.
The 7920 uses the lowest basic rate in idle and the highest rate enabled when on call.
Don't need to set 802.1p tag to 7 any longer with that code version, but is also ok to leave it configured that way as well, since 7 is still the voice queue.
Would suggest to open a TAC case to seek assistance with this issue.