Consumer Redressal

I had a connection of Tata Photon Whiz that was taken from a local dealer.It was on billing.I used to pay all the bills in time.Because of the slow speed I went to the dealer for disconnection and made full and final payment that was more than that I used.They told us that your connection will be disconnected by itself within 3-4 days.But now after 3 months Tata Photon has send us message that the bill of Rs 1217 has been generated which is unfair.I have receipt of last payment we had made.Yesterday i got a call from the Advocate of the company that you are payment defaulter and you will have to pay fine of Rs 25000.This is for what??????????.What is my fault?I want Justice as i have not used that dongle after disconnecting it with the dealer.Please help in sorting out all this matter and bring justice to us.