I believe that it should still reduce the likelihood of spam calls - because you don’t advertise where you aren’t selling stuff, and if you’re selling stuff you don’t want to piss off the local government.
For reference: where I live the “do not call” list is from the state. Most of those spam calls come from people in other states controlled by the same republic, thus not subjected to the rules of my state - and yet the “do not call” list still does its job.