Just add the number to your reject list.
And you are all correct, it is because you have posted either a rental or property of some sort on either Craigslist or Kijiji having included your phone #'s.
It has happened to me, too. /:(
Holy crap you guys too? I just got 2 calls today from that number. I answer and nothing. Call back and it's busy. Haha I guess we're on some list somewhere and this program is just running through which numbers are active.