MTNL Broadband — Misinformation of plan details

Initially on application I had requested for a night unlimited plan and assumed it was done as whenever I logged in to the mtnl account to check details of my account it would show night unlimited. Based on that we downloaded accordingly. 2 months later I get a 14000 bill stating we were on 200MB free only plan. When we took this matter to the exchange they made us produce the receipt and never in our wildest dreams had we doubted the person taking our application form but it was 200MB on the receipt. my online account now too shows 200MB. How am i to prove that we were misinformed? What can be done to resolve this? My landline is now disconnected due to this as we haven't paid the bill since the matter is not cleared. Besides don't MTNL have a credit limit which if it was effective none of these complaints would have arisen and lesser losses incurred