Q: unlocking iphone 4s

Hello,

My brother, who lives in the U.S. gave me an iphone 4s with IO 6.1.2 and firmware 3.4.02. The question is how can I unlock this phone so I can use it in my country. This phone was purchased from Sprint. My brother called Sprint and a Supervisor gave him a six digits number which according to Sprint agent is the Network unlock code. Now, is it true? And if so, how do I unlock the phone using this code?

Thanks in advance ....