Originally Posted by NegraCardio
If you put a T-mobile ROM on your AT&T device, you run a serious risk of bricking the unit. You can get a T-mobile SIM working on it by doing a GSM unlock, and you can flash any AT&T ROM you want, but Wi-Fi calling won't work with the AT&T ROMs.
Sent from my SGH-M919 using xda app-developers app
No offense but I'm sorry to say you're not right that he'll brick his device, I have an AT&T HTC One, unlocked using a code, and root it myself following the easy to follow guide available on XDA for AT&T HTC One. Also I have tried almost all 4.3 Rooted T-Mobile ROMs including Stock as well as Custom ones and have not yet bricked the device.
I appreciate all the hard work and support provided by XDA devs and mods and other users.
I installed these roms in order to get the wifi calling on my device.
UPDATE THE RADIO TO 4A.21.3263.03_10.38m.1157.04L AND FLASH AN ADNROID 4.3 T-MOBILE WIFI CALLING SUPPORTED ROM, it worked on my HTC AT&T ONE, I hope it work for others.
TIP for noobs like me, ALWAYS WIPE before flashing a different ROM/Firmware and "READ the INSTRUCTIONS".
---------- Post added at 09:42 AM ---------- Previous post was at 09:08 AM ----------
Eyezayah, can you please change the subject to TMOBILE WIFI CALLING ON AT&T UNLOCKED PHONE. There are too many wifi calling threads floating around.