If you call *2 from your Sprint phone and ask for the itnernational department they can go over unlocking procedures and all of that with you. There are requirements for the account to meet eligibility to unlock an iPhone that they can check. The intitial launch of iPhones were unlocked and then correctly locked when Apple updated the software the first time. Unless the iPhone is still using the original software it had when it was launched the phone will be locked now.
My daughter has an iPhone 4S on my plan, which she received very shortly after the release date. I recall early phones were shipped unlocked. She'll be leaving next week for a semester abroad and I'd like to be sure her phone is unlocked before she departs so she can use local SIM cards in those countries where she will be traveling (12 weeks).