At first, VoLTE will be used to deliver "Rich Communications Services" (which look like something similar to what Voice over IP/Video over IP outfits offer today) rather than completely replacing narrowband voice. It will be awhile before Verizon is willing to commit to that, due in part to LTE outages in late 2011.
On the flip side is MetroPC, which has pushed VoLTE as hard as it could.
Then there's Sprint, which will stick to voice over 1x for the foreseeable future, but routes Direct Connect over IP. Sprint's latest DC incarnation seems quite similar to one facet of RCS over LTE.
So that's a wrap on that. This merger was the best thing to ever happen to Sprint to be honest at this point. We're finally getting the network that we deserve.
Flashback (11/30/2016): https://www.fiercewireless.com/wireless/sprint-cfo-robbiati-600-mhz-spectrum-past