In response to this news about AT&T blocking robo calls. https://www.techspot.com/news/80874-att-begin-call-authentication-screen-out-fraudulent-callers.html
We don't really have a problem here in Australia, I get just a few a year, but I understand its a much bigger problem in the US.
Is it possible to make an Android and iOS app that will answer an incoming call and play a recoded message to the caller asking some questions, then listen for answers, then if the answers are correct, ring the phone and connect you to the caller?