One more time. There is no code used in unlocking an iPhone. There never has been any code used in unlocking an iPhone. I and others have unlocked many iPhones over the years. The process is, and always has been:
1. the user requests the unlock with their carrier or the carrier the device is locked to
2. The carrier the device is locked to authorizes the unlock and, internal to their company, sends that authorization to Apple
3. Apple provisions the authorized unlock in their hardware database
4. The user applies the unlock to their handset through a restore as new in iTunes.
The Sprint reps you are talking to are flat out wrong. The carrier you take it to will be unable to do a single thing to help you. This is on Sprint, and only Sprint can fix it.
The invalid SIM error message will appear with any carrier when using an carrier-locked phone on another network. All that message is telling you is that your phone is indeed still locked to Sprint and will not work with any other carrier until Sprint properly completes your unlock request.
P.S. Unless you're speaking with the CEO and the chairman of the board, there is always someone to escalate a customer service issue with when you are not getting the help you need from whomever you are talking with. If they tell you that, they are again simply lying to avoid dealing with you.