Troubleshooting No Service

Hi.. I am from Turkey,

I bought two iPhone 5 from Milan, Italy on 18.12.2012.

When I come back to my country,One of my iPhone doesnt works.

I put my simcard in my iPhone ( my gsm operator is Turkcell ) , It asks pin code and after that , It doesnt works. It says ''No Service''

and when I try other gsm operators simcards ( Vodafone Tr and Avea ) in this iPhone , it works.

and when I put in my Turkcell simcard the other iPhone 5 , it works again.

in my country,gsm operators works with different Bands.

Vodafone TR - 1800 Band

Turkcell - 900 Band

Avea - 850 Band

I think one of my iPhone 5 has a band problem.

because it works with the other gsm operators , and the other iphone works with my simcard.

Please, tell me what can I do for this problem.

Unfortunately, in my country there is no apple store and apple service.

Thank you very much.