Hathway Broadband Internet, Bangalore - Hathway harrassment

After I relocated, I checked with Hathway for service in the new location. They denied and so I asked them to disconnect my service. However, they call me everyday either asking why I am not using or for some new offer they have. This is happening everyday (I am not exaggerating) since * months and now in addition to these calls there's another additional call once in every fortnight and that too on Saturday late afternoon asking to return the device. When I ask them to call me back in sometime so that I can be either available at home or arrange someone to be at home I never receive the call back. Today again I received one such call and I asked to speak with a higher authority and would like to raise grievance against all these kind of actions. The person on call said she is the highest authority and said she would send me a legal notice. Could you please let me know where or how I can raise grievance against all these harrassment and mental torture