Airtel — Being charged by airtel when it was it's free

Airtel is charging the money during the free pool period, and the customer care says you have minus balance. My question is when they told me it's free trial for a month why are they telling me I have to pay to resume the services, Airtel is very bad at their service in Kaval Byrasandra. All those people out their in Kaval Byrasandra pls refrain from getting enrolled to Airtel, I have even complained about not getting the OTP. Even this issue is not resolved.