That already exists, it's "voicemail". The scammers never leave a voice mail (idk why). If a real person is trying to reach you, they'll either leave a voice mail or text you after you don't pick up.
In my country, despite voicemail being available since the introduction of mobile phones decades ago, I am yet to hear of a single instance of anybody actually leaving a message.
But voicemail implies storing the audio somewhere, and that means cost.
And at least in my country one should explicitly enable voicemail. I never could make it work for some reason..
And as far as I can see, it is not widely used.
EDIT: Oh, I completely missed the fact that there can be a fake voicemail where the phone automatically answers and asks the caller to speak and record it and store the audio on the phone itself. Then the user can check such recorded messages later..
Did you mean something like that? I am really surprised that this is not common already...
>Note: You can play back the audio data only to the standard output device. That is, the mobile device speaker or a Bluetooth headset. You cannot play sound files in the conversation audio during a call.
Damn google! You did this so that apps cannot do the above. You cater to spammers!
Many spammers leave a prerecorded voicemail, they call quickly from 2 numbers so they can slide into your voicemail instantly without ringing more than once
If someone calls from an unknown number, they get some sort of captcha to prove that they are a human, or they matter is important.
For example, the message should say that, if you are geniune, then please call again after 1 minute..