Deduction of amount from prepaid account without verbal consent or verbal confirmation

Your Location/city: PUNE

Name of the product/service/company:: Airtel India

Refer Mobile number 9890682781

Dear Sir,

I got a message in my inbox

You have been successfully subscribed to 1:Friends Voice Chat and the amount was deducted from my prepaid account without verbal consent or verbal confirmation. Amount Rs.30

Sir,
We never subscribed for the service

So immediately we forwarded a sms to
121 and 1210 STOP 1: Friends Voice chat
and the reply given by sms was

Sorry we can not process your request try later

We do not want any advertisements on
9890682781 by sms or by phone call

We do not want any company promo ads on
9890682781 by sms or by phone call

Please do the needful

How can they cut the money without verbal consent from the subscribers
prepaid account