Calls intermittently going directly to voicemail

[Brian]

I guess it depends on router, but am not 100% sure of the technical reasoning. What I do know is that most of the time when people report issues and we check logs, we see the traffic coming in from the remote media gateway on a port within to 5004-65000 range and the router blocking it or misrouting it. Some routers let it through, but the ones that analyze it sometimes see the connection as "unknown" since it's from a different IP on a port not already being used, etc.

I guess, since it's not within VOIPo's network, it isn't possible to narrow this range down, or specific ports? What do those of us with multiple VOIP providers/lines do?