Vodafone India - Charged international roaming charges incorrectly

I have been a customer of Vodafone for the past ** years. In these ** years, not even a single time I missed making payment on time.

I placed a request to activate international roaming in April ****. BUT IT WAS NOT ACTIVATED and I suffered a lot during my travel because of that. In the bill generated in May ****, I was charged for international roaming as well. When I raised a complaint, VODAFONE checked with their backend team and confirmed that roaming wasn't activated. After that, they rectified the bill amount and I cleared the remaining amount. In June ****, I ported my number to airtel. Now, the bill generated by vodafone added back the incorrect due of international roaming again to my June **** bill and is forcing me to pay the amount for the service which I never used. I raised a complaint ** days ago but it has not been resolved yet. Meanwhile, I have been receiving messages from my new service provider airtel that I need to clear the due with Vodafone to avoid my number being barred. This is insane behaviour from Vodafone that it never bothers to resolve customer's issues but still informs airtel about the pending payment.

I have never used the service and how can vodafone ask me to pay for the service which I never used? Vodafone's backend team already confirmed in May that roaming wasn't activated. Why is the amount added back to my bill again in June then? Why am I being blackmailed through messages that my number will be barred if I don't clear the amount?