how do i lock a "unlocked sprint phone" to sprint

I use sprint and my old phone broke so i bought a new sprint SPH A900 phone off ebay. But when i called sprint to activate it and gave the ESN they said they couldn't activate it. I went back to the ebay page i bought it off of and it turns out i bought an unlocked sprint phone. (i didn't know phones could be locked and unlocked). How can i lock an unlocked sprint phone back with sprint??? Someone please help me out.

The problem may have nothing to do with your phone being locked or unlocked. The problem may be the phone was reported stolen or lost, or maybe the previous owner didn't pay their cell phone bill. In those cases the ESN will be listed as "bad" and Sprint will not activate it. You may be out of luck, and whatever cash you paid.

Well i thought about that too..... before i found out about locked or unlocked phones. When i called sprint it sounded like the ESM wasn't in their system at all. They kept on asking me if i was sure it wasn't a prepaid phone.... and made me repeat the number like 10 times. I got my bother to talk to sprint and they told him i might have a virgin phone or something. Also when i got the phone it was brand new... everything still in plastic with no finger prints or anything, the phone was in perfect condition. If it was a used phone and was lost or stolen then i'm sure they would have told me that when i called.... "Sorry we can't activate this phone because it has been reported lost or stolen". A few years ago i had lost my phone and the sprint representative told me that when i tried to active a phone my brother had found. Also when i press *2 to activate it like the manual said it trys to connect me to cricket. Its a sprint phone..... all i want it to do is connect to sprint. WTF

ok i just found out from the guy i bought it from on ebay that the phone i bought was set up for Cricket service. Is there still any way to get it on the sprint network?