Consumer Redressal

I had subscribed for 150 Rs internet pack for my newly ported airtel postpaid no. 9019775166 ,I had consumed 460 MB 3G data (less than allowed 500 MB) and 40 Mb 2g data(less than allowed 500 MB )but I received a bill of round INR 1400 Rs/- for 4 days of internet usage .While initial inquiries they also agreed that I have used the same amount of data (mentioned above) during my billing cycle.But after I explained how their calculation was wrong they started saying I have used the mentioned data apart from the allowed data(500 MB of 3G and 500 MB of 2G).None of their Customer care executives/representatives have been able to justify the bill calculation ,I received a call regarding the bill clarification but it got disconnect after incomplete explanation .All of the above clarifications and conversations have been through e-mails.I would like to file a case against Airtel for first making a mistake in bill calculation and then giving delayed and unsatisfying excuses to their customer.