There's also the quick and dirty way. Honestly, this is one of the reasons I've rarely used a custom greeting in Audix.
Just set your vm-box to use the "system greeting".
If you're away, or have SAC activated, the caller hears:
<recorded-name> is not available....
If you're on a call, the caller hears:
<recorded-name> is busy...
Unfortunately, if you're busy on a bridged appearance of your extension, the caller only hears that you're "not available". (That's probably what an executive wants, but doesn't work so well with bridges to TransTalks, etc.)