I tend to agree with IP Guru. I want a back up for any IP Office with full time AA.
I build a new hunt group with every user in it (My current favorite name is Yikes). Then under ICR/Destination, I make Yikes the fallback extension.
Now, if voicemail or the server has any issues, then inbound calls will ring everywhere, and everybody's phone will say yikes. No lost calls. Quicker knowledge of the problem and hopefully quick correction.
Good luck,
Frank