Something I've found to be somewhat effective, when I suspect a spammer, is to answer and not say anything. If it's a robot caller it waits to hear someone and then starts the recording or kicks you over to a person. If it hears nothing it gives up and marks the number as a non human number and quits calling. If it's a legitimate caller they'll eventually say "hello" and then you can answer. Although it still may be a spammer.

It's not 100% but I've significantly reduced the robo callers doing this. Your mileage may vary.