Pennytel.com the Australia based voip provider is offering 2 cent(AUD) per minute rate to India that too with per second billing. Thats the cheapest one can get. Rates are available on all its plan even the prepaid one where credit never expire.
It may be noted that company charges 50 ct transaction charge on each recharge done through credit card. So if you are already having a Sip ATA voip adapter or already know how to use voip then do try this operator.