AT&T introduced VoLTE roughly a year ago, and it has gradually expanded since. The ability to talk over an LTE connection has come to more markets and devices alike. But support isn't ubiquitous in either area, and hardware that doesn't ship with VoLTE out of the box needs a firmware update to make things work. Now it's the LG G3's turn.
The update bumps devices up to software version D85021l. According to the changelog, it also provides AT&T Mail and Uber. You can check now to get the goods over-the-air, but if nothing happens, you may have to wait your turn.