A few people have mentioned they'd like this service, where a random digit/puzzle is presented. I decided to focus on people who wanted the most extreme call blocking service but would like to come back to this later.
Just like account security questions. Provide drop down selector/s for users to select their question.
I'm not going to provide my public number here, but if a caller of mine isn't whitelisted I just prompt them for my billing zip code. Now, that's not going to work for every one, but considering who I want to get phone calls from, it's pretty foolproof.
Keep the energy up @davidajackson. I'd love to see you hugely successful with your efforts!