With the Android 4.4.4 update already filtering out to devices, it's a wonder that the Android 4.4.3 update is still rolling out to some devices, but that's what we have here as of yesterday. Sprint's version of the HTC One M8 just started receiving the rollout of the 4.4.3 update, which basically brings some bug fixes and a few features.
On top of the Android 4.4.3 update, Sprint has also thrown in some extra goodies. The included an HD Voice icon and WiFi calling! That's not too shabby!
It probably makes sense that they would just release this update as they were probably working on it longer than the 4.4.4 update. We expect that to hit the device within the next 90 days.
Source: Sprint