Airtel — Disconnected a long time ago, but the company demands money even after 6 months

Address:

North 24 Parganas, West Bengal

Hi..I am Subhendu from Kolkata....I took broadband services from Airtel along with a fixed line service on June 2011. After 4 months i sent a mail to them on 13th Oct 2011 requesting for discontinuation of the service. I received their acknowledgement mail on the same day and one reply mail on 15th Oct 2011 saying "As per the telephonic conversation on your mobile number 983****5** ,we would like to inform you that your request for cancellation is registered under the reference number 5***5** and same will be addressed by 7days". I have even cleared the dues in Oct, 2011.
After that I exchanged the service with BSNL. NO AIRTEL LINE WAS ANYMORE EXISTING IN MY HOME. I didn't had airtel broadband as well as airtel fixed line since then.
But I kept getting their monthly bill along with regular phone calls from them demanding the money. When I mentioned about my discontinuation of the services they said "okay, it may not have been updated in our system". Now suddenly they are calling me again and demanding money saying I have dues about Rs. 2000. When I informed them about their reply mail of Oct, 2011 along with reference number of my cancellation of the service, they said that I must have some "Docket Number" but later agreed to give me 19% of rebate on the due amount. But why should I give any money to them as I had received their mail with reference number of my cancellation i.e. my services were discontinued from Oct 2011 ? And how would I know that I need to get "Docket Number" from them as a proof of cancellation ?
They are threatening me now saying that they would go by legal procedures and I agreed on that. Rupees 2000 is not much and I could have given that to them to avoid further hazards but I decided not to give that money to them as after discontinuation how can they demand any money? God knows if many others are suffering just like me.