If you are going to the US, I would just not recommend this option! It is terrible! Although they give you an AT&T SIM which works great, the problem is when you get billed once back in India.
It is simply cheaper to rely on Skype on a smartphone to make calls from Airports. Once in the country, just walk into an AT&T store and get yourself a GoPhone prepaid SIM. You will be happier and incur very much lesser bills.