what's an esn number..[/QUOTE] An ESN Number....is a serial number that identifies your specific phone out off all the rest on the assembly line. You will typically find a sticker inside the phone under where the battery seats. It will either list it as ESN or S/N.
If your phone has a clean ESN you will have access to use the phone. If the phone DOESN'T have a clean ESN, either the phone has been reported the phone as stolen or....something to that effect. Buyers beware...if you are buying the phone from some place like Craigslist, be sure that the phone has a "clean ESN" before buying.
First off, you canflash a 4G phone to cricket; currently you just disable the LTE feature and tell the phone it's 3G only. Also sometime in the not so distant future cricket will start offering LTE service.