Airtel - Wrong billing for more than 10 months

My Current Airtel postpaid bill plan MYPLAN FAMILY CORP *** CUG ** -NAT offers free call benefits of *** minutes + other benefits.

This plan is discontinued in Airtel from Jan ****. But since i am old customer they are supposed to continue giving me the plan. As they cannot change the existing post paid customer plan.

Since this plan not exist in Airtel system or there is a bug in their billing system i am charged full for every minute i speak. i have *** mins free but if i speak *** mins it calculates for *** without subtracting from *** free minutes. i have to send mail every month from Jan till November. i have to explain this to *-* customer care representative. Half of them will not understand what i am saying. Every month i have to followup ** times to get my excess amount charged. Now for November month they are reluctant to refund now. They are saying pay full. i asked them explain how i am changed more. else i wont pay my bill. But no reponse i have sent * emails so far. i am frustrated to the core. Seems to be some issue in Airtel billing since my postpaid bill no longer exists. After multiple follow up i got every month refund. But now this month they are reluctant to refund. Even to get ** months refund i had to mail follow-up explain ** representatives. i am frustrated to the core. i want to solve my billing issue once for all. i cannot follow up every month like this.