One of the early proposed uses of digital cash-like systems was for spam reduction in email by charging a fee for every outbound message (could be refunded for legit callers). An advantage over a captcha is it doesn't use introduce friction for the caller by taking her attention.
It's weird, I'm on google play store but there aren't any apps that seem to filter calls based on audio captchas. There are other apps that seem to programattically do some filtering--it seems crazy that someone hasn't already done filtering by audio captcha.


"Nasty little Buddhist"
Seeking via neuroscience and psychology informed dharma.