https://support.google.com/voice/thread/3022858?hl=en
Updated July 8, 2019
The scam:
You put your own personal telephone number (mobile or landline number) out in public somewhere (on a classified ad, or a dating website, or wherever).
Some scammer contacts you via text or email about your ad. They tell you a story about how they need you to prove you are real person, or a legitimate seller, not a bot, and that they are using a special phone service that requires that you give them the six-digit code number that will be played to you by an automated verification call or text message you will receive from Google.
The scammer is, in reality, going through the Google Voice setup process. They tell Google Voice to call your personal number, and then the call speaks the code, or the text message supplies the code, along with a warning to not share the code with anyone. Somehow, you ignore that explicit warning and give the scammer the code number. When you do that, THEY, not you, are issued a Google Voice number, using your personal number as the forwarding number for their account.