Airtel — incorrect bill accepted but not refunded to bank

Amount not refunded for incorrect bill to bank-

Airtel accepted they had generated an incorrect bill of more than rs 4000 /-. They said they will adjust the amount in future bills. As i want to disconnect using airtel connection, i have called them almost 10 times in the last few days to as for refund in my bank account - no response. Everytime its downtime for connection or they promise to call back and never call.

I am trying to get the refund for incorrect bill since 20 august bill generation. They have already charged for september bill when i didnt even want to use airtel anymore and was using another card whole time.
Really frustrated with the service - airtel is not disconnecting my connection and keeping on paying the bills by the refunded amount.