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.
At least on Android it seems this isn't possible for 3rd party apps to access the in call audio source.
https://developer.android.com/reference/android/media/MediaRecorder.AudioSource …
This means you can't process the audio stream for a response like a dial tone. Seems to rule out audio based captchas.
What a bummer :(
@Android


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