Based on my VOIPo experience, when callers get the "out of service" message, it is due to VOIPo's servers not being able to talk back to the database servers due to latency, outage somewhere, etc. The upstream carrier doesn't get a response in time with what to do with the call, because the database can't be reached to see what to do with it (fail it over, voicemail, etc).

Something like that anyway!