Why isn't my phone working abroad?

I purchased an HTC Sensation from t-mobile four weeks ago in the US and requested an unlock code. I entered the unlock code, then took the phone with me to India, where I replaced my t-mobile SIM with one from Airtel, an Indian carrier. I called Airtel to provide data services, which they say they have done. But my phone is unable to connect to data. I am only able to make / receive phone calls and texts, but no data. Help??