From what I understand, Sprint just refuses to unlock any phone. Sprint doesn't actually unlock any of their phones. They unlock them only for use outside the USA, even though they're all 100% compatible with USA GSM/WCDMA networks, even some LTE networks.
So like BMWxi said, your only option for now would be to pay a 3rd party to unlock it. Common thing of prices being under $50 for a unlock for sprint are usually shady/a scam, since its essentially getting a 3rd Party, paying a portion of the $50 to a Sprint Employee, for a code or sim card to unlock the phone, afaik
From: Silentcore
First of all hello since I haven't been here for a while. The thing is that I got an Iphone 5S which has a clean ESN and everything but the previous carrier was with Sprint. It's "locked" to sprint and for some reason I can't seem to activate it with other carrier until I unlock it. Does anyone know something about this and maybe give me some tips or something like it. Thanks in advance guys.