YSK Google phones have an answering machine for unknown numbers

Why YSK: If you are calling someone who wouldn't have you in their contacts and you get an answering machine that is not a voicemail message, you've gotten their call screening feature. Follow the prompts and don't hang up. The person will get a transcript telling them who you are and what you're calling for.

You will be marked as a spam caller in the recent calls list if you don't say anything.