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.
No it’s not required. My 24 month contract was almost up so paid off the two months I had left and had it unlocked just to do some testing on a few other carriers with the esim. Atm my esim has T-Mobile Prepaid. And getting about the same download speeds of between 100-130 mbps on both Sprint and T-Mobile. Uploads are usually around 5mbps on Sprint and about 9-12 on the prepaid T-Mobile line.
Sprint LTE Speed Test
T-Mobile Speed Test