I have bought an iPhone from usa.

Hi,
I have bought an iPhone from usa. i have unlocked it. i was able to use some sim cards in usa. but when i try to use airtel in bangalore, it is saying the error message. ” iPhone can be used only with an approved sim card.” anybody faced this problem can help me? b’cos of this i am not able to use my phone for the past 20 days.
pls. help me to resolve this problem. thanks for your help.
Mobile: 9900922541