The problem is, zero of the carriers are offering support for VoLTE on Windows Phones, not because it's not there, but because they don't sell them (okay you can get it on a branded 950, from AT&T).
This means, if you're like me, and have a really strong Band12 tower nearby, and a sort of junky Band4 or similar, you get sketchy voice calls, because you can't use the good tower/antenna/band.
Carriers don't seem to want to budge on this for Windows phones either, they're locked in, or at least playing all their cards really close. Either way, as they continue to expand the low-frequency bands (i.e. 12/700, 17/700, etc), our performance will get worse, and they don't really care, because they only building-out the newer tech.
My AT&T data is great, my voice, well, it works, mostly. I drop from 3-5 bars to 1-2, when I make a call, and I definitely can't use data at the same time, with any consistency, very frustrating, particularly when they could simply provision the fix.
I don't think MS and the perception on Windows Phones' future is exactly helping the cause, either.