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.

You must take your personal number back, away from their Google Voice account.  Detailed instructions are below.

You do that by (re)adding your personal phone number to either your own existing Google Voice account, or, if you don't have a Google Voice account of your own, then

You need to create your own Google Voice account, and add your own, personal phone number to it as a forwarding phone number.

Go to https://voice.google.com and, if you have never created a Voice account, click the "Get a Voice number" link, and then follow the instructions to get a Google Voice number.  When asked to link a forwarding phone number, don't enter your number that the scammer stole.  Instead, enter some other phone number, at which you can receive one verification call from Google.  It can be any non-VoIP home or office or mobile number, which hasn't been previously-used to obtain a Google Voice number.  You can remove it later, after you recover your stolen number.

This time, do answer the verification call or read the text message, and then enter the six digit verification code from Google.  Never do this under any circumstances, other than when you initiate this procedure yourself.

After your account is set up, go to this page:  https://voice.google.com/settings

Click the rectangular box, that says "+ New linked number" to link another number.  Now, enter your scammed phone number as the forwarding phone you wish to add.  If the reclaim process for this number hasn't been used repeatedly in the past, you will get a warning that the number is currently in use on another account (duh!), and would you like to reclaim it.  Respond yes, of course. This will remove the number from the scammer's account.

Note:  if you do not get a warning that your number is in use, this simply means that the scammer had already removed it from their account, and you now have nothing to worry about.  Please don't post more questions, asking if you need to do anything else.  Your number is safe, and you are finished. Understand: your own security has not been compromised; this scam only leveraged your personal phone number as an “admission ticket” for the scammer.

Please be cautious of fraud when interacting with strangers on online sites, especially on Craigslist, which is a frequent target of scammers.  Never, ever share verification codes with anyone.  No legitimate business will ask for that information.  This is an example of the clear warning to not share verification codes with ANYONE.