Steve,
They're all correct. It is from Craigslist, I'll tell you exactly what happened.
C-list changed their policies on posting multiple ads from the same account.
Many power users who post 100+ ads a day therefore must have multiple accts.
Now C.list requires each account to be phone verified.
Meaning you need a verification # sent by c.list to your phone to activate each acct.
Google: 'Craigslist PVA's' see what I mean
You'll see dedicated websites selling PVA's for $6-$13 each.
C-list says 1 posting per acct per 48 hrs, and no cross posting to multiple cities.
So imagine those power users need lot of phone #'s for cheap,
and this verification # he was hoping you'd pass along to him.
Power users even change their IP address, use proxies, to get around the rules.
Not a scam. It's craigslist verifying your number for post. It's only a scam if someone has used your number to register a posting. At the end of a new post, craigslist asks for a phone number to verify your post. You have the option to recieve a phone call or a text. Then to finalize your post, you put in the "code" instead of the email verification craigslist use to use.