I have a theoretical solution that would work without being dependent on phone number block lists (which change all the time or can be spoofed) and doesn't let spammers/scammers leave a voicemail. It would also still allow good robocallers (like school closures, etc.) to leave a message.
The problem is, based on some quick back-of-the-envelope math, I'd probably have to charge $20/mo. to make it worth it. In my experience, consumers just won't pay that much. They want things to be free.