how to unlock iphone 4 version 5.0.1 model MC318LL

Hi

Dear

Dir/Madam

          I am Ferdinand Panganiban from the Philippines, I own an iPhone 4 version 5.0.1 model MC318LL which a friend gave me from California,however an issue arises since the phone is lock to a certain network (AT&T) sim.. meaning i cannot used the phone to other network here in the philippines.So, I can't make phone calls or send or received messages. More so, there is No SIgnal or No Network coverage of the phone even if I have used a sim card used by Networks here in the Philippines.

          Hoping that this matter be given due course.

          Thank you so much.

          Kindly reply. You can reach me through this site or email me at fhrdzz@yahoo.com / +639264204570