The X55 modem can do VoNR (Done on T-Mobile with a OnePlus 8). I would expect T-Mobile to add VoNR via software update to all X55 phones once they have SA and VoNR ready on the network side.
They are trying to get two different results with the upgrades. The 600 Mhz upgrade was to get more range from the site to cover fringe areas. The 2500 upgrade is to get faster speeds & capacity in major population areas.
One thing I do like about T-Mobile is that they really put a lot of energy into taking care of Houston. T-Mobile pulled a bunch of permits from the City of Houston on June 10. Seems like they're not sitting on those permits (which Sprint used to do).
From the few towers I checked, it seems like they are prioritizing towers that were left behind during the 600 Mhz upgrades first. The towers that received the first round of 600 Mhz upgrades have not had any permits submitted.