T-Mobile is starting to focus a little more of their efforts towards the support of their rolled out VoLTE network, which at this point consists of Seattle, Austin, Atlanta, Chicago, Dallas, Houston, Long Island, Los Angeles, Minneapolis, New England, New Jersey, New York, Philadelphia, and San Francisco. Those are just the officially announced locations where T-mobile has turned on the VoLTE network, and as of now to accompany the rollout in those areas, T-mobile has four phones that currently support that network. The previous three that T-Mobile had included were the LG G Flex, the GalaxY Note 3, and the Galaxy Light, and soon users of the T-mobile Galaxy S5 will begin getting the update to support the VoLTE network in those listed areas as well.
The update is scheduled to begin at 5PM, so at the time of writing this people should already be getting updates. As with any other software update this one will take a little bit of time before it reaches every single T-mobile Galaxy S5 user, so although T-Mobile is rolling out this update as of now, not everyone will have it before night's end. That is the most likely scenario anyway. The internal document that mentions the updates lists the following: "Beginning June 19 after 5 p.m. PST the Samsung Galaxy S 5 will have a software update to Android 4.4.2/Baseband G900TUVU1BNF6 that turn on VoLTE services via OTA over Wi-Fi in available VoLTE markets only. For devices not in VoLTE markets, the update will be available via Kies starting June 20. VoLTE capabilities are limited to markets that have had VoLTE turned on.This update provides features and improvements"
So there you have it, as listed by the documents from T-Mobile themselves, you won't be getting the update via OTA if you're not in a VoLTE market. However if you're not in a VoLTE market just yet, the Samsung kies software will be your best friend starting June 20th, so it sounds like users will still be able to get the update they need to support VoLTE either way, and it's just a matter of a couple of days. Users should also consider that even though they have a two day wait to get the update through kies while others in the VoLTE markets can have it starting today, some people may not get the OTA update for a few days so you may end up being able to get it before those actually live in a VoLTE covered area.