A:Answer From what AT&T explained, if LTE stays on the screen next to the cellular signal icon at the top of your screen, then it's using HD voice, which requires the LTE data. It is not HD voice if it changes to 4G.
You'll need to also check with AT&T to see if it is available in your area. As of right now, Gotham City (Ok, the St. Louis, Missouri area,) does not have HD voice available.
Make sure you have LTE cellular data on for both calls and data to ensure you're getting HD quality. If you find you're getting dropped calls, or calls that sound funny, you probably don't have HD calling available and turning LTE data off for calls will improve these conditions.