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.
Yep, no real reason to add new towers in most areas. TMo's network is spaced for AWS at this point in urban/suburban areas so they have their pick of where to put 600 panels. Merely swapping 700 for 600 on the sites that have 700 would be an immense improvement.
Likewise, if M-MIMO n41 is as good as folks say it is, AWS LTE spacing should be sufficient for that as well. Particularly since TMo can pick L-Sprint sites if those are better. Depending on area, those sites are spaced for either PCS or B41.