Broadband company threatening for non payment of bills what to do

I had been using reliance broadband for 5 years. In last year April, it stopped working. After series of complaint, they told be it could never be resolved. Therefore, I made a request for disconnection in June, and they finally disconnected in August. But, a month later, they sent me a bill for May, June, July and August. I explained everything clearly but they never listened and asked me to pay. Later, I got a couple of calls where people identifying as lawyer from Delhi consumer courtand threatened me with law suit and arrest if I don't pay. I refused. Today, I got a call from a person identifying himself as a lawyer from consumer court in Delhi. He threatened me with dire consequence and sent a SMS stating I have to pay 15000 fine in court. My doubt is 1) Are they really lawyers who call me? 2)Should I be worried if they send a legal notice? 3) How should I handle if I receive such calls?