First of all you need to know something. Only the carrier (Sprint in your case) can unlock the phone. They locked it in the first place. In addition, Sprint will ONLY unlock the phone for international use, not domestic. If you want to use the phone with a US carrier, you need to purchase a new phone. The Sprint iPhone 4S and 5 can not be used on any US carrier other than Sprint. This has not changed since the introduction of the iPhone 4S on Sprint.
Accourding to official documentation on Sprint site, here is the circumstances when Sprint will unlock your device.
- The device is Domestic SIM Unlock capable
- The device must be or have been active on the Sprint network for a minimum of 50 days
- Any associated Service Agreement, Installment Billing Agreement, or Lease Agreement has been fulfilled including payment in full of any applicable early termination fees or end-of-lease purchase options. Because leased devices are not owned by the user, the end of lease purchase option must be exercised and paid in full before the Domestic SIM unlock is completed.
- The associated account is in good standing
- The device has not been reported as lost or stolen, associated with fraudulent activity, or otherwise flagged as ineligible to be unlocked
So, if you meet all the requirements, contact Sprint and they will unlock the device.