I just had "almost" the same thing.
Customer said that when people call in it does not go to the AA but to another phone (just like your VM).
I called in to test and it works just fine.
I had the customer test it - works just fine.
Next day they called back and it does not go to AA.

So I went to site checked around and this is what I found.
The receptionist phone was set for "FWD ON BUSY" YES
And it was set for this other phone.
Possibly just like yours may be set to FWD ON BUSY to VM.

When I test called - she was not on the phone - works great.
Cust calls and she is on the phone, then it goes to the wrong place.

Something to check for.

If CON is the opposite of PRO
Then what is the opposite of Progress?