Thanks to an XDA developer we now have a T-Mobile ROM that has been modified to work on AT&T LG G2. This will allow your AT&T phone to work just like T-Mobile’s LG G2 thus giving you the ability to use T-Mobile WI-FI Calling.
Then reboot and that is it. Your LG G2 now supports WiFi calling.
There is a slight bug in this ROM where the phone does not wake up after double tapping the screen. For that:
Go to the phone dial pad
Then select Update Touch Firmware
Wait about 30 seconds, and then back out to the home screen.