So network busy is due to QBSS. QBSS is also part of 802.11e. If you look on the 7920 site survey display, you will see CU (channel utilization). If this is >= 105, then will get network busy and not allow outbound calls.
For this type of AP, it could be tied to WMM being enabled. Could try disabling WMM, but if the channel is busy, then quality may suffer.
Maybe can try another channel as well.