My experience tells me it varies by network. All my family runs on ATT, Verizon and TMobile.. and all get spam by robocalls. Meanwhile I have SimpleTalk (which is a rebrnd t-mobile so you get reception of tmobile network under silly name simpletalk), I got about 10 robocalls since I have this number circa 2011.

Also - certain area codes are hotter than others. While I have Cali number, I know many folks living in Alaska and never got robocalled.