Airtel — Payment accepted for wrong account number

Airtel migrated me from copper to fiber on their own and also allotted me a new account number.
At the end of billing cycle they sent me an email with link to make payment for the old account. The last & final bill.
I clicked on the link and made the payment. I was under the impression that payment is done and everything w.r.t. old account has been settled.
However since my mobile number is linked with now new account, they eventually considered the payment as advance for the new account.
I requested them to adjust this amount for old account as it was intended for. They are not saying anything on it now.
They do not respond to emails. I called customer care and raised an SR. They silently closed the SR in the system without any resolution.
This is a classic case of lack of accountability, lack of integrity and lack of customer centricity.