AT&T WiFi Calling On Google WiFi does not work when two iOS devices are on the network when DNS is set to Automatic. Change the setting under Advanced Networking/DNS to ISP DNS. Then Forget the WiFi network on your iOS devices and rejoin the network. Turn on WiFi calling under Cellular settings on the iOS device. It should the word WiFi should appear immediately in the iOS status bar.
Unfortunately, this method only works for a short while. So, I replaced Google WiFi with DLink Deco. It works flawlessly.