Hello,
Your project looks really interesting for me. I can put a Kamailio and RTP proxy in front of your system, so you'll have a "topology hiding" for outside world and no one will realize what's happening under the hood of your system. You'll need an additional server with static "white" IP address on it, you can take a cheap VPS for that purpose.
Ping me if you're interested. Thank you.
BR,
Dmitry H.