My suggestion (if it's possible):

Assuming that the PSTN timeout is set at 5 seconds, where if there's no valid response within that time, the upstream carrier gets a 'network unavailable' error code and plays the disconnected intercept...

Could Voipo's front-line server/incoming call gateway (assuming that's not the point of failure) have a timeout of, say, 4.5 seconds, after which it plays back a "There appears to be temporary trouble with this line. Please hang up and try your call again." message? I'd much rather someone calling me have to try calling me again than have them get an incorrect message indicating that my number has been disconnected! I know - ideally, there's no (OK, very little) latency within your network, but it would be a nice backup feature. That is, of course, assuming that you would not be able to have your system follow the failover instructions in the user's vPanel...