VOIP systems don't really work like random webservers though.
Instead, we're locked into proprietary platforms or relying on old phone systems.
No, there’s a reason software like FaceTime, and calling in general, are mediated by centralized services and implemented as client-server. Because relying on your users to have stable IP’s, and to always be behind a properly configured firewall, would be an utterly insane way to design something.
Hence “ipv6 is a relic of a bygone era”, because in the era we live in now, IP addresses change and users move between networks constantly, and NAT is very much the least of our problems.
You're pretty much never getting a new IP at every cell tower.
> Then I get to my friends house and his router is gonna block unestablished connections so you better call me soon.
PMP can solve that.
> Because relying on your users to have stable IP’s
Not a stable IP, but some kind of stable identity. Like a DNS name. That can be provided by someone like Apple, or Google, or whoever. It doesn't need to be ultra-centralized by only a single thing controlling it.