I forgot to mention that I used my number in UK between 2008-2010, and I assume that both India and UK are using same GSM networks. I guess US has some different networks which may not be compatible with Indian sim cards?
Don't know about iPhone, but in an android phone, you can open the SIM toolkit (Vodafone Services SIM Menu) and then select international mode (in Roaming Option).I have a Vodafone prepaid service, after talking to customer care, I have recharged 1000 and 673 for roaming. But still I see no service on my mobile. I called customer care to help configure my mobile, unfortunately they do not know how to guide for iPhone. Could you help me set up international roaming on my mobile?