Seems similar to Hamachi. HamachiX is a Hamachi GUI for OS X, otherwise it's command line. Hamachi automatically traverses NAT firewalls and I've played "LAN" games over the Internet with it. You create a private network with a password and people join it. Now anything that's able to work on a private network works perfectly like it was a real network in your own house.
I'm not sure if this will work for sure but if your software does SIP calls directly peer to peer then it probably will.