Help with Sprint iPhone5 unlock

[mjoshi123]

Okay if this is against forum policy please delete this, I've iPhone5 and iPhone4s both out of contract from Sprint. All Sprint will do is provide me with unlock codes but nothing more than that. I've got codes from them for my devices. But I cannot use it on other networks. When I put in SIM Card it gives me message stating "Please insert another SIM CARD from a supported carrier or request that this iPhone be unlocked by your carrier." I'm trying to see if I can use it on Cricket Wireless -which is AT&T MVNO. Anyone know a means to do this ?