In my experience, it does exactly what it says and has already saved me lots of time and frustration. It’s not that I often answered spam calls; I had already trained myself to almost never pick up a call that my phone didn’t associate with a contact. But I still had to stop what I was doing and look at my phone or my watch whenever one came in. Now that’s a thing of the past.
I just realized I hadn’t enabled this feature yet, so I went ahead and did so. I pay for Nomorobo to block spam callers, but for some reason I’ve been unable to refresh the app’s block list since updating to iOS 13. Hopefully this new system feature will work well enough that I won’t have to pay to block spam callers anymore.