Airtel — one time charge in postpaid bill

Address:

Nagpur, Maharashtra, 440001

I got a post paid plan of 299 RS for number [protected].
For two months I received a fair bill. But this time I got a bill with RS 300 extra as one time charge and my total bill amount is 659.658.
I am very disappointed for such hidden charges. Why do a costomer will choose any plan if company will charge any cost according to their benefits.
This is definitely Not tolerable anymore.
I want my bill to be revised. And should be without any such extra charges.