Deduction from main balance after exhausting netpack limit

I was using prepaid BSNL Netpack of Rs 56 of 250 MB for 30 days validity but on the last day when I was using net, the netpack limit exhausted and without disconnection of net or any intimation in form of SMS, Rs 38 got deducted from main balance.
Amount is not much but I want to complain about the wrong way of deduction from main balance without intimation