I was just reviewing my call detail records on my "home" number yesterday, and realizing that the vast majority are spam - we pretty much don't answer that phone. This got me thinking about alternatives to even ringing for any unrecognised numbers, such as an IVR menu that said something like "Hi, we're screening spam, press 4 2 if you're a real person". I'm not sure how that'd be accepted by, for example, the dentis…
It has a whitelist/blacklist for common recorded messages like dentists, and then a CAPTCHA pretty much exactly what you describe: https://nomorobo.zendesk.com/hc/en-us/articles/205761825-Wha...