Airtel — Fraudulent billing for more than fifteen thousand rupees

I have been an Airtel Black postpaid customer for more than 10 years.
Last month they incorrectly added international roaming services worth more than Rs.15, 000 that I never requested or used. Airtel call center agents accepted that it was their mistake. But they are not reverting the incorrect billing. In parallel, I keep getting calls from their collection agents to pay the bill. I am not an able to port my connection because of the outstanding bill. This is mentally harassing.
Human mistakes happen. I can understand if some agent by mistake added a wrong item to my account. But what I am not able to digest is what happened subsequently. The complete and systemic disregard of a customer's complaint is something I never expected. I have sent at least 10 different emails, called their customer service 12 times, and also visited their physical store so far. Every time they accept their mistake, register a complaint and then silently close it without resolving.