BSNL Broadband — Excess Billing of my broadband internet account

I have been using internet in a very limited access and for the first 2 -3 months of my usage the bill was noramal with Home 250 plan . I had actually paid the whole years bill in advance. But in bill of September month i was shcoked to see a bill amount of about Rs.3700 and i had also made the payment to avoid any late charges. I had not used the net that much nor did i download any data . I used to only check mails.I had written letter to the Divisional Engineer Ambattur telephone exchange but as usual no reply. This month again the bill for the month of Octoberthe bill value has come to Rs.6800. Please suggest me as to how do i take action for this wrong billing from BSNL.