Airtel — higher postpaid bill

Address:

201014

I have opted for rs. 499 postpaid plan and in the first month the bill is at around rs 500, but from second month onwards they are increasing the billing amount every month without any clarification and it had reached at rs.900. Most of the time i use wi fi connection and hardly use my mobile data.
I need to know how they calculate the billing amount and my detailed transactions history including everything.
And in case if there is something illegal going on there should be very strict action related to resolving customer complaints, their loss and punishing the culprit.