4S not activating (model no: md235f)

Hi guys,

I have huge problem and I couldnt find a solution.

I received a 4S 16GB black as a gift from my cousin. He bought the Iphone from France with Bouygues operator. But I want to use it in Turkey.

I dont know if it is simlocked or not. But when I try to activate it with Turkish GSM card, it is not accepted but signal bar and 3G is working. By the way, I could activate it by Boygues sim card then inserted the Turkish GSM oper. card (Turkcell ) , the name of Turkcell apprared in options >about > operator. Then it said it needs to be activated. but it couldnt. I also upgrade ios into 5.1.1 . how can I solve this problem? is it really locked ? how can I be sure if it is locked or not ?

thanks in advance

yavuz