Customer has been cheated

Location/place: Hyderabad

Name of company/service: Airtel Broadband

I had booked an broadband airtel connection in September 2011. They had charged me Rs.800 for the instrument. As the installation was free they had charged nothing for that.But during the installation the executive had given me an old instrument which was badly used and not at all in a working condition. However, he had installed the line and promised me that he would replace it in a day or two. But its almost 8 months they haven’t replaced the instrument and when I have reached out to the customer care they have replied back to me saying that it would be solved within 24hours. But again it didn’t happen and the worst part is they keep saying that an executive has visited my place and he finds the instrument to be in a perfect condition—which is absolutely rubbish. I have even cleared the monthly bill with a hope that they would solve this problem, but it didn’t happen. Now when I have stopped the payment, the executives call me saying that since your issue is resolved you will have to make the payment.
Iam very clear on this now I don’t need a new instrument but I would like to sue them for the service that was delivered to me and for the false promises. Kindly advice.