A month back autoprime , a recognized Contributer at XDA posted a working SIM unlock method for Sprint variant of Galaxy S4 – SPH-L720.
You can contact Sprint for unlocking your Galaxy S4 for use on GSM networks internationally, but many users at XDA reported its not reliable. Even after Sprint unlocking devices, popping in international SIMs did not worked for many and many can’t even contact Sprint ( bad ESN etc )
In this topic posted at XDA, autoprime mentioned the prerequisites and steps to unlock your device. Major requirement your device needs to be on MDC or MDL baseband. This method of sim unlock will not work with devices on MF9 baseband ( steps to downgrade the baseband are also included ). Be aware of the fact that you still can’t use ATT and T-Mobile SIMs after successfully unlocking( only international SIMs work)
Head over to this post at Androidfannetwork.com if you have a Sprint HTC One and looking for working sim unlock method.