AT&T rolled out a massive update to Galaxy S6, S6 Edge Plus and Note 5 with build numbers G920AUCU3CPHA, G928AUCU3CPHA and N920AUCU3CPHA, respectively.
The update brings WiFi calling, Advanced Messaging and September security patches for Android.
The update is pretty good to install, but if you care about rooting your device, know that you won’t be able to do so after installing the PHA update.
It’s a very rare case scenario that an AT&T devices gets rooted easily, and even if it does, OTA updates tend to patch that root exploit and make the device unrootable again. So if you have root on your device and you don’t want to lose it, it’ll be best to not install OTA update and rather wait for someone knowledgeable over at xda or other Android communities to patch the OTA update with root and make it flashable via TWRP recovery.
That said, WiFi calling is a pretty cool feature and might be more useful than having root access (for those who keep root for fun). Decide for yourself.