I wouldn't hold my breath waiting on this thing to get certified for VoLTE with T-Mobile, especially this late into the game. The only Sony model (unless I am forgetting one) to ever get VoLTE and Wifi calling certification from T-Mobile was the XZ1 Compact. It had this feature from day 1 because it's something that has to be cooked into the OS. Basically Sony got on the horn with T-Mobile and worked out all the nitty gritty details before it was launched. VoLTE isn't something that a device manufacture can patch or enable on their own. It has to be done on the carrier side too along with FCC certification to be sold that way. Hence why if a device doesn't launch with VoLTE don't ever bet on it getting it in the future. Especially with a new device coming out every year it's just easier to put it into the XZ3 should they choose to do so at this point.
As for Band 12 you don't need VoLTE to use it. You can have access to Band 12 as long as your phone has the radio to support it. So basically you can get the benefits of 700 mhz spectrum for browsing the web and watching videos indoors etc but it wont help you on actual phone calls. Anyway hope this clears things up.
