Sim not supported message when a Airtel sim card is inserted

When I insert a Airtel (India Carrier) SIM card into my Iphone 7plus which was bought in USA, I get SIM Not supported message. After I remove the sim card I can access the phone features and so when I go to Settings->General->About, it shows Carrier as Airtel 32.1, so what is going on with this phone? I did buy a unlocked phone so not sure why it says SIM Not Supported message.