Airtel — airtel - changed validity of prepaid amounts and forcing to pay extra to continue services

I wasn't told about any validity before and now saying to recharge with minimum of 35rs every month to stay connected. I have enough balance in my account. They never told about any validity date and now suddenly texting and calling to ask for extra amount to get continued services else the amount already paid would get wasted. This is injustice and incorrect.

Please review these forceful and dominant plans and take necessary actions.