A subdomain can be easily redirected, but I think VOIPo would need it on their DNS just like I set usa2k.com registered at Godaddy to the HostGator DNS Servers. I am not expert in DNS though.

So I would expect the URL would need to be tied to DNS servers at VOIPo.
I guess that means all resellers would be on one DNS server, and that would be a clue ...

Perhaps there is a Hosting Guru that can explain how this should be done?