Harassment by MBD Inc

I have never had an account go bad but I am being harrased on a monthly basis by MBD Inc. I am receiving threats that my Credit rating will be affected for not taking their calls but when I answer there is no response. I've tried blocking them but they call on my work number. I found a number to contact them on but the people that answer stay quiet and then cut the call. I need these people to leave me alone or if someone can please tell me how I can report them.