We once built a service like that as a joke on a Twilio hackathon. I've solved my robocall problem by enabling Do Not Disturb mode. Only people that call twice

We once built a service like that as a joke on a Twilio hackathon. I've solved my robocall problem by enabling Do Not Disturb mode. Only people that call twice with the same phone number within one hour go through. My friends know and if someone else calls and it's urgent they will retry. But it nicely keeps all those robots away, because they usually randomize numbers.