Stop calls on my mobile number from a particular area code and exchange

I know it is possible to block individual numbers on an iPhone.  However, I am getting multiple calls a day from Area Code 857 and exchanges 391 (Boston), 396 (Brookline), 410 (Boston) and 418 (Boston).  The last four digits are always different.  There is never anyone there when I answer the call.  I receive about four to six calls a day and it's really annoying.  I'm on the "do not call" registry.  Anyone have any ideas on how to make it stop?  Thanks!