If you are recharging your Tata Docomo Mobile number using their website, you have an option to Pay the recharge by Mobile balance. Pay by Mobile balance option helps to be able to buy a special recharge online without using your credit/debit card or net banking account.
I have posted on getting FREE 500 MB Data from Tata Docomo and pay using balance was the option explained.
Please note that this option is not available to all recharge denominations. It is not working specifically for the denominations mentioned in the post Rs. 71, Rs. 79 and Rs. 171.
Is there a connection?