Bharat Sanchar Nigam Limited [BSNL] — Adjustment for rs 500 in bill has been coming

Address:	Muzaffarnagar, Uttar Pradesh, 251001
Website:	portal2.bsnl.in

I have been getting a fixed bill of rs 1139 every month until last month, on which i have to pay extra rs 500 amount (Called as adjustment). There is no information what this extra amount is accounted for. Showing adjustment doesn't clear any thing. It had happened 2 months before as well in which i gave around 450rs as adjustment. Please clearly state why this extra loot is being taken from us. I strongly disagree with this and want a real adjustment in bill by removing this 'adjustment' amount. Please find below account details:

Customer id [protected]
Account number [protected]
Invoice number [protected]
Invoice date 06-apr-2016