I had a conversation with Verizon Support on Twitter a couple days ago. The short answer I got from them was as of Sept 30th this year, any unlocked 4g phone should work on any US carrier. This is the first I've heard of this. Has anyone seen any press releases or other are there Verizon people here who can explain more about this? It sounds like a really cool thing! I'd be advertising this if I were Verizon!
There are different VoLTE bands. The ones that the 950/950XL are set to operate on are not ones used by Verizon Wireless. In the future mobile phones should be able to hop carriers so long as the correct bands are supported.