What do you plan on doing with all your existing T-Mobile subscribers?
Eventually they'll need to be moved off the T-Mobile network, but that's roughly 18 months from now, nearly an eternity in this industry.
That said, T-Mobile themselves are set to retire their 2G network around the same time period anyway, so with the expectation that most devices will move towards VoLTE as the new default, it just becomes a matter of finding who has a VoLTE VZW-compatible on our network currently on T-Mobile (most iPhones, newer Samsungs, Motos) and shipping them a SIM card when we're ready.
I have found VoLTE requirements to be a major pain in the butt.....
(This comment is not aimed at Ting in any way other than the policies of the carriers they use.)
The issue for now is that for many phones, it's nearly impossible to find out whether VoLTE will be supported on either Verizon or T-Mobile networks without the actual device in hand, and in some instances actually activating, or attempting to, activate, it As far as I can find, neither VZW or T-Mobile makes this information easily available, beyond a list of "approved" phones covering newer, common, and more expensive, models. The situation is likely to get worse before it gets better.
For now, the situation actually pushes me towards recommending Sprint or AT&T networks for many friends-- a lot less unpleasant surprises that might result in unacceptable performance, and a lot more likely that their existing phone (or a budget used purchase) will actually work.
I don't doubt that the industry wants to consider 18 months "nearly an eternity" regarding phone ownership, but I'm certain there are plenty of folks here using a phone older than that.
I don't disagree with you on T-Mobile's VoLTE requirements. Even some phones with LTE Band 12 don't support it, and we're finding that out the hard way in support.
As VoLTE becomes the standard, I think it's a little premature to assume that no budget devices will come out in the next 18 months that do support VoLTE on all US networks, as Verizon themselves stopped activating non-VoLTE devices months ago and more T-Mobile users are finding out that their 2G areas have been refarmed into oblivion and are unable to make calls without VoLTE compatibility on the towers and phone in question.
As to old phones -- that's sadly a fact of life, I think, in this industry. We'd like to think all phones are future-proof, but as they're really only designed to have a 2-year life span (with some notable differences *cough*Apple*cough*), it makes sense that the Big Four would make moves like this when most of their retail customers would be likely eligible for an upgrade to their handset at that point anyway. Remember, we play in their sandbox by their rules.