Right after boasting its wideband LTE and VoLTE networks, T-Mobile has turned on voice over LTE (VoLTE) for a few devices. The devices come from both Samsung and LG. The update to enable VoLTE is appearing on the Galaxy S 5, Galaxy Note 3, and G Flex.
Technically, the update only matters (right now) if you are in one of the 16 markets that the carrier’s VoLTE network has been launched in. Until then, just enjoy the update and wait until your market gets VoLTE.