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.
A full deployment has never been T-Mobile's strategy, and that's why they've never been the best in Seattle. Despite having the best site density (by a lot), their network experience falls far short of AT&T, Sprint and even Verizon now, who I would have claimed was in last place a year ago.
The trend for the last 4 years has been that AT&T and Verizon never climb a tower without deploying every LTE technology available at that time. Even today, the same cannot be said for T-Mobile.
I would estimate that more than 35% of T-Mobile sites in Seattle are still midband only. And some are still B2-only whereas others are B4-only. That makes coverage/capacity inconsistent between sites and handovers at the edge of cell sloppy, to say the least.
To really compete with the big two, they're going to have to rethink the way they're deploying their RAN and stop deploying the minimum needed to get by. Hopefully we see those changes going forward, because they definitely have the economies of scale necessary to do so now.
Don't forget that the US Government paid AT&T $6.5 billion to deploy B14 FirstNet spectrum. AT&T took advantage of those subsidized tower climbs to upgrade its own infrastructure to 5G-ready, and add all of its spectrum holdings. Smart move.
T-Mobile doesn't have that subsidy, but I agree that any tower they're going to touch from here on out should be fully upgraded, and I think we'll see that with the Sprint "keep" sites for sure, plus anywhere they add NR equipment.