Airtel — postpaid connection - overcharged for data, but i still have 23gb data balance.

30gb free data was offered 5 months back for my postpaid mobile connection and i had accepted it. However, from past 3 months, airtel is charging 500rs for data per month while i still have 23gb data balance in my postpaid account. I called customer care in oct and 500rs was reverted back. But in nov and dec bill again 500rs has been over charged per month. Till now i have called atleast 4 times to customer care with recent call done on 26th dec 2017 with complaint reference number 82262667 and the customer care person by name deepak had promised to resolve it within 48hrs. Till now it is not resolved. Airtel customer service has become pathetic and looks like they are least bothered about customers. Airtel will end up losing customers with such carelessness. I am frustrated calling customer care very often without any resolution. Please help to get out of this trouble. I have decided to move out of airtel once the billing issue is resolved.