Most of IP-based server services (daemons) allow to specify the IP address/es to use, chosen, obviously, between the IPs the operating system (Windows) offers.
I have done that for SMTP servers, FTP servers, SQL servers, Oracle servers, etc. I'm assuming VM server listens on all IP addresses. A shame, but still the IPO
unit fails to connect to all IP addresses (one IP at a time).
Where this "Backup voicemail address" setting is? Note that we are talking about IP Office/VoiceMailPro, both Release 5. I know about VM backup server, Release 6
and later. Not the case.
To answer your question: both main and "backup" servers are running Windows Server 2003 R2 with VoiceMail Pro Server 5.0.30.
Regards.