Airtel - Transparent Extortion by Airtel

I had visited Europe in September and had taken the Airtel Global plan for Rs. ****/- approx. During my visit, Airtel automatically hiked the amount by Rs. ****/- which was unethical but I let that pass. Upon my return, I paid the full charges for Rs. ****/- approx for which I have the Airtel Receipt with Transaction ID as well as my Credit Card statement. Now Airtel has again added that amount to my next month's bill and made my life miserable with repeated calls followed by threats to disconnect. THIS IS PURE EXTORTION AMOUNTING TO CRIMINALITY. Big companies cannot be allowed to harass retail customers like this. Nowhere in the world, it happens like this!