Sprint clean esn phones can include wimax 4G, LTE and 3G. LTE is not working on most but Ting has it. Wimax is working on some.
Some prepaid divsions like Boost Mobile and Verizon prepaid allow activation of certain handsets from their postpaid/parent carrier. Boost mobile is limited to under thirty select handsets. Verizon prepaid, most 3G phones will work including the iphone. No 4G.
2) Flashing. Reprogramming phones that are not supported by the mvnos like non Verizon phones on a Verizon mvno (fairly easy), LTE devices from Verizon on a Verizon mvno. Verizon, Sprint and other major carrier phones on an independent regional (Like Cricket).
Flashing requires technical skill, software, hardware and runs the risk of bricking the phone or getting only partial functions (talk/text only, 1xrtt data only, etc). If you're unsure, stick to the first method of activating clean esn postpaid carrier phones on prepaid divisions and mvnos that run under their system.
3)Rewriting esn. This allows any cdma phone from any carrier to work on any carrier but is fraught with legal grey zones and black zones. If a person who's flashing is able to "magically" put any cdma phone on a network that doesn't allow byod/byop (Virgin Mobile for example), then it's been rewritten. They take a legitimate serial number from an original phone that belongs to the target carrier and it is grafted onto a phone from an otherwise incompatible carrier so the system believes that said grafted phone is one of their own original ones.
Talk of how to do this used to get people banned on Hofo but I believe they've relaxed the rules on that just a bit.