Fraud by tata photon.

Location/place: mundra

Name of company/service: tata docomo

Hello, I have been duped by tata photon, as a customer care executive called and approved my new bill plan to 6GB unlimited plan, but after few days when I had used more than 6GB of data, I get a sms that my credit limit has crossed, after that I called the customer care service , they say my new plan will be activated from 20 sept, while in my account details page its showing that the 6GB UNLIMITED plan is being activated. Now they are not willing to accept their fault and is saying that I have to pay the amount . Somebody please help. some ideas.