Being billed 2 months after disconnection

I placed a request for disconnection on 12th November 2014 saying I want to be disconnected on 19th November 2014. I was told that this is too much notice.. contact us on 16th. I contacted them again on 17th November.On 20th November, they gave me a Cancellation number. I let them know that I had already left Chennai on 19th November and so they should proceed with the disconnection. In December, I contacted them, received a pro-rated bill, paid it by 19th December (before the due date of 28th December, 2014). Now they send me a bill in January for another month;s services. I have written to them multiple times with copies of all emails.

Nobody at Airtel Customer Service reads the emails. Now they are threatening to send me to a collection agency.

I DO NOT OWE ANY MONEY TO AIRTEL BECAUSE I CONNECTED 3 MONTHS BACK!!!!

I should not be harassed like this. I NEED YOUR HELP.