@OP : are you using GPRS or 3G services by any chance? Please remember that if you're using any service in a scheme (like for example, the airtel 2gb for 98 rs plan), the amount will keep accumulating and will show as an unbilled amount. My 'unbilled' amount comes to 5 or 6K in the GPRS section by the end of the month. Once the regular bill is generated, the amount is placed in discount ( i.e discounts -5000 rupees), and I get the 'normal' usage amount. Please do confirm with customer care and check your final billed amount.