Virgin or any other MVNO that uses Sprint's network cannot activate a Sprint phone because it would breach the MVNO contract. Sprint only allows Virgin and their other MVNOs to activate ESNs within a certain range. I did some more checking into this for you to find out why. Nobody wants to breach a contract with Sprint or they might have to hit the road and find another carrier to lease airtime from, which would be a service refund nightmare and possibly the kind of thing that could bankrupt some of the smaller MVNOs.
Also, despite what anyone tells you, tampering with the
ESN to make the phone work is a very bad idea, it's likely the phone would roam all the time, drop a lot of calls, and you would be unable to use text messaging or the internet. Besides, that is very complicated and difficult to do... and if not done exactly right you are left with a useless paperweight.