One option (at least for the VM part) either have the computer run a watchdog program on the vm server or have the IPO set to send alert emails (or SNMP traps) when there are problems. That would cover the majority of your possible problems.
The other more obvious problem you have is hearing busy if voicemail is down. Ideally you would have a fallback incoming route to a human (or group) if the VM is not available.
A fun way to monitor your inbound lines would be to setup a "Wardialer" to dial a special DID and respond to different ring/busy/answer scenarios.
Or for a hefty monthly fee, I can call your system all day and night to let you know it's down.
Or....since I'm in Houston also, could help on a professional level if needed.