Airtel - Bill plan changed without my consent

I have been using postpaid airtel services for more than *-* years. I chose a family plan of Rs *** earlier which included * calling sim and one free data pack sim (**********). Airtel employee gave this free sim as it was a kind of free service that airtel was offering its customers with that family plan, however, there was not any such requirement of this sim from my end. 

Each month I received a bill in the amount of Rs ****.** including GST for my postpaid plan. But for the month of July ****, the billing amount came to Rs ****.**. After checking the billing, I noticed a few unwanted changes in the bill.

I was charged Rs ** as an add-on number charge for that free data sim which I never even asked for and I haven't used it since the day I got it as a free sim. In addition, the bill plan has been revised from *** to **** without my confirmation. 

I tried several times to file the complaint using airtel thanks app but to my disappointment, it didn't work at all and every time a message popped up that stated there is an error currently and asked me to try again later.
On ** August, after so many attempts, I finally got a chance to talk to airtel customer service agent through the *** complaint number. After discussion, the employee transferred my call to a senior official named as Mr Vishal Saini who introduced himself as a senior customer care official. He said that the company has sent messages to inform customers about this change on just the primary number. I asked him to file my complaint as I was not satisfied with his reply. He denied doing so and asked me to send an email regarding the same.
I sent an email regarding this issue. No one responded to that. I did tweet about my problem by tagging Airtel Presence. They replied that they'll call to resolve. But nobody called and they are just replying through DM since so many days that they apologize for the delay and asked me to pay the bill. Also, gave assurance that they'll adjust the differences. But it was just a message. No one called to discuss and resolve the problem.
It is very sad to see such poor customer service from the company. How could they change the bill plan without taking confirmation and permission from the customer? I chose the postpaid plan of Rs *** earlier as it suited my needs. How could they decide on their own that this financial change in the bill would be okay for the customer? it is me who has to pay the bill, not the company. So the right to choose a billing plan is mine not theirs to decide on my behalf without talking to me and without my confirmation for the same. Moreover, when the billing amount is decided, the company send the bill on all the numbers in this plan and also through email but for this major change they thought that only sending messages as they send other promotional messages to the primary number is sufficient.
Although I am completely dissatisfied with the bill as they changed it without my knowledge and permission, I am ready to pay the bill according to the changes as I used the calling services but I can not pay the charges of that add-on as I have not used the sim at all since day *.

Kindly help me with this.