I bit the bullet and upgraded my phone to the beta. Wi-Fi calling is working in Miami, FL!
As a network admin by trade I got curious and did some sniffing around. It seems WiFi calling is implemented using an IPSec tunnel. From what I can tell (it's encrypted, so you can't tell much) it's tunneling IPv6 over your IPv4 WiFi. If you open Network Tools (from Hurricane Electric) you can see that your phone will have a couple of IPSec tunnel interfaces with world routable IPv6 addresses.
Basically, AT&T is doing VoIP in its truest form, giving everyone an IPv6 (whether native on cellular, or through a tunnel when on Wifi) address, and routing calls and text messages over that.
There's a lot going on, and a lot that can go wrong, which is why they are probably rolling it out to beta users first and not to everyone at once.
As a network admin by trade I got curious and did some sniffing around. It seems WiFi calling is implemented using an IPSec tunnel. From what I can tell (it's encrypted, so you can't tell much) it's tunneling IPv6 over your IPv4 WiFi. If you open Network Tools (from Hurricane Electric) you can see that your phone will have a couple of IPSec tunnel interfaces with world routable IPv6 addresses.
Basically, AT&T is doing VoIP in its truest form, giving everyone an IPv6 (whether native on cellular, or through a tunnel when on Wifi) address, and routing calls and text messages over that.
There's a lot going on, and a lot that can go wrong, which is why they are probably rolling it out to beta users first and not to everyone at once.