Kasul Sheshagiri

Air tel is charging very amount for the 3G and their plan says very simple Rs.240/- for 1 GB for a month. If you disconnect they will charge you prorate basis which should be less then Rs.240/- but for my surprise they charged as they promised i.e., less then Rs.240/- but they charged Rs.5300/- for data usage!!! There is a big difference in the plan and the amount they charge. What to do in such cases? where we have bill and we have all the records. How to deal with such cases?? Can any one help me??