Airtel — Harassment from Airtel for Due which was cancelled in Nov 30

I am an Ex-ServiceMan 83 years old and getting harassment calls from Airtel. I had a data service from Airtel for 4 months and did pay for that upfront. I took the service in Sept 2021 and cancelled on Nov 30 as I did not need it anymore.
Note: I had already paid for 4 months which I am not asking for refund

I got an confirmation email from Airtel that the service was cancelled.

Now ( in March) after few months, Airtel is calling me to say that the service was never cancelled and asking me to pay until March. I can forward all the email chain.

Need your help to resolve this issue with Airtel