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.
The 123 subway station at Penn Station finally has B41. Its from the Transit wireless DAS but in the past it only had B25 LTE. Nice to see more B41 lighting up underground. I'm going to check the ACE side sometime this week as well.
I'd just wonder how long it would take to get the networks to mesh. How integrated would they need to be to have seemless handoffs, especially with voice. Then how long will this integration take to get to that point. That's if they even integrate networks. Maybe they could say just point Sprint's equipment to TMobile's servers, push some updates and be integrated. Not knowing a thing about how their networks work, this is the grey area and the area I'd be concerned with.
For data, I can see just open up bands to each others customers, retransmit requests/packets if hopping networks. But not sure that will go well for VoLTE. Plus there could be a lot of lost packets which wouldn't be good for a lot of services. I'd imagine at first say as a Sprint customer I would be able to use the TMobile network, but when the phone scans it would scan for Sprint channels first then scan for TMobile if it can't find Sprint.