well you can go to consumer court but they are like any court and will cost money as well as time [ though not as much], go on the offensive now , i hope you have documented every email from them regarding this, land up at the nearest head office of airtel in your city and bang the regional manager's head in, on a side note hire a lawyer[ this will be cheap] just to send them a notice of wrong full billing and also casing harassment , BEFORE you get legal notice which you will, but the legal notice from airtel hardly amounts to anything as it will be a civil notice the proceeding in itself can take years just to start , but if you send a notice to airtel their legal rep has to present it to regional board and they generally avoid these things as you can really sue them and just in case if they try collection guys then you can make it a criminal matter of harassment , airtel would avoid it at all costs. all in all a messy affair , best bet talk to top guy from airtel in your city.