They call and call and call when I answer NO one talks if they get voice mail no ones leaves a message

These calls are coming in from the 906 area code - and are from a security firm.  I didn't get the full name of the company, but they have called me three to four times a week.  When I asked them to put me on their "do not call list," they said they couldn't because I was a business, then hung up.
Since then, I still get their calls which I ignore.