Tmobile India

TMobile in Chennai India is completely unpredictable. 
I managed to get some data by connecting to Airtel, but it is impossible for me to make any calls to US numbers. Says I don't have ISD. 
All these partnerships are very unclear and even Tmo has no clue and most agents won’t be able to guide you. 
so if you are coming to India assuming it will work, please know it is a hit or miss. And if you think TMo customer service will help you - THEY DON'T KNOW EITHER