A lot of the spam calls I get are just recordings or they just hang up once I pick up. Hard to imagine it would help to just say "please add me to your do not call list" into the void...
The internet myth is that this marks you as someone to keep calling (because you listened), but really they will stop.
I mean, I get ~1 unwanted call a week across a couple of phone numbers (that I answer and ask to stop). Other people are saying that they get a dozen a day and giving up on their phones. My anecdotes work for me.