Thank you for link.
Have you ever done IP phone vpn-in to the server? The overseas part they have fast internet speed over there, so my concern was latency and call quality. Do you think this setup should work and provide clear voice?
Technically it could work, but always depends on connection. You would need to troubleshoot connections issues (latency, jitter, etc.) with the Asterisk tools and Wireshark, if you even have them. Guaranteed minimum connection speeds can be a good way to avoid issues on the external side.
iperf -c other.end.ip.address # that makes it the client
After 10 seconds (by default), the two consoles will show you what you can work with.
Repeat the process vice-versa.
Try it, for ease of use (but perhaps more confusion) use ’ -p 5060’ as the transport and -u as the protocol on both server and client. That might help diagnosing firewall/natfups