Four years before I had same problem. It seems still they have not improved. They charged RS.700 for 20 mts of internet usage. I had registered complaint. No one bother to address the problem. It dragged for three months. In the mean time, I had torture from their customer care executives. Three to four call per week for this pending payment. The joke is, I used to pay other charges (Voice usage, rental, etc) regularly except this dispute charge.
Finally I want to come out of the service. As a punishment for being Tataindicom customer, I payed Rs.700/. I know it was kind of looting. But I paid just for my peace and come out of their services immediately. The same statement I put it in my termination request.
If you want real peace, do not go for this service.
Their service is being recommended only for the people who loved to being tortured.
Latter I learnt from my friend that he had similar problem with their billing system. He proved the same by running some scripts in his front end to capture his internet session details. He produced the same CDRs to the Tataindicom Central office and he was successful to revert his additional billings. As he is a techie person, he was able to prove the same. Think about others.