I have been called by 303-217-8653 and I just did the most sensible thing:  BLOCKED them with my cable provider's block service.
Boo-ya! Seems like they are from Colorado. I have also eliminated the 'anonymous' by using 'anonymous call reject' which forces them to divulge who they are or the call gets rejected.  .  Every single one of these b*****ds will get the same treatment, and when they call on another number, a harrassment charge will be filed with the phone provider per FCC rules.  Don't sit down and take it.  Fight back with what your phone company (or provider) has to offer you.  Use the harrassment/threatening complaint mechanism and you can get the phone company to monitor your line.  These jerks WILL be fined, and they WILL stop.