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.
So I’ve heard two different things now. First I heard before today that on day one it would be reciprocal network sharing on day 1 which is obviously not the case. Then I heard today two somewhat conflicting statements. First that for now users will stay on each separate networks. Then I heard another statement that Sprint users would be able to tap into the T-Mobile network via expanded roaming. Can someone clear up what exactly has changed today if anything?