It's actually quite simple. Both phones have IPv6 and exchange them via Nostr. IPv6 gets rid of most of the NAT stuff. Now all they need to do is to open the firewall.

For that, we can start with a server or a DVM somewhere that people can activate to open each phone's firewall. But after the first few are open, the phone's themselves become STUN servers and start openning connections for everyone else.

There is always a good amount of people online. The idea was to use Nostr's network effects to keep punch through the ISPs firewalls.

I think it works. I just need a lot more testing.