Airtel Prepaid Service
Unauthorised deduction of balance

I have taken internet service pack of Rs 98 on 19/08 which is valid for 1 month usage with 1GB. On 30/08, my balance was Rs 22 and 639MB left (internet surfing). After net surfing, I disconnected the internet & got the message that Rs 14 has been deducted as a last call charge. I have called the customer care two times, but instead of replying anything, they disconnected my call & send a reference number via sms (Ref. no.-CHTDE20831140054 & CBDCC20831141429). I want to know why on what basis they have deducted Rs 14!!