Idea Cellular Limited - Monthly plan changed without our knowledge

I deal with my ** year old mother's phone(idea postpaid connection) which had a monthly rental of Rs **.( My mom has difficulty walking.) Suddenly on *th Dec, ****, they changed it to monthly rental of Rs *** as RS ** plan was taken off market.we realised this when we got the bill in Jan ****. When I called to enquire, they blamed and was so adamant that we changed it through a website, for which I kept saying profusely that we didn't do it and don't need to as my mom uses a feature phone. I told them to take this as a fraud and follow whatever procedure they had to get the amount refunded. Several phone calls and emails were made but no resolution other than they just telling that we did it from a website portal and that too at * am( as if I didn't have better work to do at that time)...In between all this, my mom went to their store to get the connection changed to prepaid as *** was too much a rental. And after their ridiculous time period and procedures, finally it has been activated today by recharging RS ***( mandatory or else they can't activate the prepaid connection it seems)...All this while I had to pay the postpaid bill as well with the monthly rental of ***