I learned recently about ZRTP-encrypted video calls using an SIP client, such as Jitsi or Linphone or Blink. I was able to make a direct encrypted video call between two clients on the same local network, but it seems a lot more difficult to directly call between clients on different networks without a registrar service.
I understand that no software has such a feature due to the complexity of directly connecting two nodes that could be behind NAT, firewall, and other network complexities. But is it worthwhile to try to make this work? What are the possible security benefits/issues? Are there alternative options (VPN, setting up own registrar/server) and how to configure them?