The "network busy" message is due to a high channel utilization issue and would prohibit outbound calls.
However, your CU value if correct is very low.
The CU comes from the QBSS feature which would be enabled when WMM is enabled.
Can try to disable WMM and try it out.
But would recommend this if other clients are using the AP as well.
The 7920 was never supported with the AP541 and the 7920 reached end of support many years ago.
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