Airtel — Airtel Postpaid generate over bill of Rs. 3349

Hello.. Myself Vijay Parmar, I had purchase postpaid connection for my father for monthly rental of Rs. 120. with free voice call, SMS and 30gb/Month mobile data.

I had paid last three month bill of Rs. 120, but suddenly in this month of July-2021 my father's airtel profile show his bill of Amount Rs. 3349, while complay give credit limit of Rs.2000 how can they charged over amount bill without our consent.
what kind of action i have to be taken to rectify this problem..

Plz help me out.

Regards,
Vijay Parmar