There's no "unlock code" for a CDMA phone, that's for GSM phones that are locked against SIM cards from other carriers. The "lock" in a CDMA phone is the serial number of the phone being in the carrier's database and, since yours was sold as a Virgin Mobile phone, its serial number is in Virgin Mobile's database, not Sprint's, so it's impossible for Sprint to activate it (they can't even bring it up on the activation screen).
There are ways of using an old cheap Sprint phone to "clone" your phone into a Sprint phone, but a post here long enough to explain the process would probably get deleted because of its length (if it weren't deleted because it's against forum rules or something). And the odds are that you'd end up with two dead phones.
There are people who will do the work for you - look on eBay.