To add to Omnitech's post, while Verizon has announced their intention to introduce LTE-only phones by late 2016 Sprint has not made any such announcements. And Verizon has pushed back the launch a second time: they had originally said late 2014, which then became
H1 2016.
And even if Verizon were to go all in on LTE tomorrow, there's no guarantee they would open up voLTE to phones they don't support. TMO hasn't, and AFAIK, neither has AT&T.