It takes effort to make NAT that bad. Something jank like UPnP has to map the port, and even that requires action on the device's part. I don't quite understand your SIP example; how does the router know which IP is the phone's?

Bad IPv6 is as simple as not having a firewall or default-allowing it like AirPort did. If Apple can make that mistake, a lot of random manufacturers can too. I can see the whole NATless firewall setup but wouldn't use it for a very long time, to let things settle.