Airtel — unusual billed amount than plan amount

Address:

Bangalore, Karnataka, 560100

Bill date: 13 nov 2017
Client number: [protected]
Mobile no : [protected]
Bill number:[protected]

I am currently using 399 rs postpaid plan and for last month usage i was charged 1069 rs for using both data and voice,
Data was charged extra 500 rs as my current plan had 2 gb free data and not sure why i was charged extra 500 rs with the bill when i had free data of 2gb in account.
Kindly give me a clarity on this issue at the earliest.