Generating fraudulent bill

I took the airtel broadband connection on March 2013. My Account no is 7006161770.
When I initially took the service, I took the plan in which says, if I pay the rent of 5 months in advance, I get the 6th month free. My rental plan was Rs.550. So, at the beginning, I paid Rs. 2750 (rent for 5 months).
Initially, I received the introductory letter, and also on May, I received the bill for the taxes on 5 months rent, i.e., Rs. 340. I paid that too, via netbanking.
And the Executive at the Airtel shop in City Centre, Saltlake, Kolkata, told me that I can take the same plan again after completion of 6 months.
After completion of th 6th month, no one communicated nothing to me. My email id, mobile and mailing address, everything is updated at their system. Airtel started me charging Rs. 550 rent on per month basis. I didn’t receive any communication for the bill in my email id or mobile no or at my mailing address. They kept on charging, which I was not all aware.
Then started the call from their collection agent. I told him my situation and also told them, that I want to pay rent of 5 month in advance and want 6th month waiver plan. I dont remember the name of the person. He told me he will get back to me after sometime, but he didn’t. I tried contacting, but he didn’t respond.
Then on January, a lady called up and asked me to pay the bill, I explained my situation to her as well. She told me that I have to clear the dues first, then she will be able to activate the plan of 5 months rent and 1 month waiver. Also she said she has provided a waiver of some amount around Rs. 700.
She sent a represetative on 11th Jan 2014 to collect cash of Rs. 1233, which she said clears my outstanding. And I issued a cheque of Rs.3090 to the representative as 5 month advance rent (so that I get one month waiver). She also promised that she will activate the plan from December 2013.

The plan was to be activated from 11th December 2013, and will remain valid till 10th June 2014 as per the plan which means with I don’t have to pay a penny till 10th June 2014.
After January, I was not in Kolkata, I went out of station. When I came back few days back, I found that I am receiving bills for every month and Rs 550 + taxes is being deducted from Rs. 3090 every month, which means she has not activated the plan as she promised. Instead she collected the money in advance from me on behalf of Airtel.
If I had to pay every month why would I pay fraudulent company like Airtel in advance, I would prefer paying every month bill before last date and keep the money in bank.

But till now, I am receiving calls from customer care executive regarding the bill, that I have an outstanding amount of around Rs 1500 to be paid. Why should I be paying the money again for the service when I have already paid once in advance?

Please look into these and resolve the problem asap.

Thanks
Tuli