MTS / Sistema Shyam TeleServices [SSTL] — Wrong generation of bills even after request for cancellation of account

Address:	New Delhi, Delhi
Website:	www.mtsindia.in/corporate/about-sstl.html

I bought a postpaid device of MTS in Jan, 2015. I did all the payments on time and correctly, but in April end or May starting of 2016 I lost my device and complained about the same to a local office of MTS. They told me to call the customer care and ask them to discontinue the connection. I did so.
Later, they asked me to pay the bill for April which I did and I asked them to stop the connection and the operator told me that the same would be done within a week or so. but the next month I received a bill again of Rs. 1100+ and that my connection had been barred due to non-payment of the bill and I sent a mail this time to cancel the account as I did not use it any more as the device was not in my possession and I had already asked for the cancellation of connection.
The reply from MTS customer care was that I had to pay that amount and then they will discontinue then i talked to an operator of customer care and he told me that he will see and resolve the case but nothing was done. As they already stated in the mail that they had barred my connection they should not have generated the bill for next month. But they did not do so and later sent me a bill of Rs. 2211 (for both months). I did not bother to reply now because I sensed their illegal motives. but later i started getting calls from different mobile numbers stating themselves to be from legal department or advocates. i told them the whole story as it was but they started abusing. these calls became a regular thing I was harassed by MTS for something that was illegal on their part.
They are after me now and am in full spirit to fight them in Court if they so wish.
Any other suggestions?