If someone gives me their iPhone?

If my friend gives me an iPhone that is Sprint but I'm on an ATT plan, will I be able to use it? And I have a smartphone plan already but its HTC. Would they make the iPhone plan more or will it just stay the same way ($15 a month extra plan) like I'm paying for my smartphone now?

Need it unlocked

The iPhone would have to be unlocked and the data plan would still be the same cost!