We always love it when company executives take to Twitter because it usually means they are sharing some relevant bit of intel on future devices or future updates to devices. We have an example of the later for you folks today. T-Mobile's Senior Product Manager Desmond Smith Tweeted what you see above. Apparently, the Nexus 6 for T-Mobile is very close to getting an update which will include both WiFi calling and VoLTE.
Previously, HTC’s Product Manager Mo Versi, shared that Google’s next “major release” would be Android 5.1. If you consider that T-Mobile’s own deadline (Q1) officially ends Tuesday, March 31st, and put two and two together, it's easy to conclude that Android 5.1 is likely coming in March.
How's that for a round-about way of getting there?

Source: Twitter - T-Mobile