Can a Sprint Iphone 5S be unlocked?

Can it be unlocked for an international carrier? A US carrier?

What would be the best way to do it, if possible?

They are unlocked for international use, at least the verizon ones ares. For local use, the companies have a list of all phone ID numbers and if it isn't on that list, they won't activate it. Unlocked for local use phones have all kinds of problems.

Is the phone payed off? If you bought it full price, and don't owe anything on it, than they will unlock it for international and domestic use. You have to call their international center for them to do this, no one else at Sprint will do it for you, and they will try to tell you that it's impossible.