Calling someone from an AT&T line with a compatible phone should result in much improved quality, thanks to the new HD voice feature. The update also brings battery life enhancements, which is always a welcome addition to modern smartphones.
The update will rollout to HTC One M8 handsets in the coming days, and you should get an OTA notification when the update is available to you. You can also go into the update menu and try to trigger it manually. Also be aware these updates are mandatory on AT&T. If you postpone too many times, the phone just updates anyway.
The official changelog for the Android 4.4.4 update on the HTC One M8 for AT&T is as follows:
- Enables HD Voice
- Updates Android OS to Android 4.4.4
- Security enhancement
- Bluetooth enhancement
- Adds AMBS Cloud Messaging Integration
- Adds Copy and Paste camera feature
- Adds Usage Manager App
- Improves battery performance
- Updates AT&T Ready2Go
- Updates Visual Voicemail
source - AT&T