I am a AT&T mobile customer, I used to call to INDIA, but this month I received a bill about $ 1500 only for July month , I really surprised , I was unaware that if I use calling card to INDIA, they charged me this much. What should I do now? And what happens, if I don't pay that amount, anyway they disconnected my phone.

They not only disconnect, they collect, in the most delightful ways. YOu did not read the fine print, my friend. Either that, or you talk a lot.