Craigslist has a big spam problem to deal with; anonymous e-mail based accounts make spamming very easy. They've dealt with this through phone verification -- if you try to post to a high-spam category, or your account is otherwise suspicious looking, you get prompted to enter a phone number and take an automated call or SMS with a code to put in the site. That phone number gets tied to your e-mail and can't be used to authenticate another, so spamming now requires throwaway phone numbers (harder to acquire) instead of throwaway emails.
I haven't made any attempt to test it, but they probably block using VOIP and other non-fixed phone numbers too. There are APIs available to check whether a number is one of those types, so, say, you can't sign up for a bunch of numbers through Twilio and use those to spam Craigslist.
There's a way to get around the phone requirement, too. I've seen this as far back as 2010.
You pay to post a fake job (something unskilled that will get a lot of applications) on a site like Simply Hired, Indeed, etc. Applicants come to your fake careers site. Applicants fill out the application for the non-existent job and are asked to input their phone number to verify their application.
When they input their phone number, the backend of the careers site submits that number to Craigslist. The applicant receives the call from Craigslist which reads them a number. The careers site instructs them to enter the number to confirm their application.
A few days later your site auto-emails the applicant saying that you're sorry, but the position has been filled. They forget about it and no suspicion has been raised.
For the price of posting a single job, you can get hundreds of phone-verified Craigslist accounts working.
Maybe they changed it for this very reason... I'm not sure. At the time, there was no indication that the call was from Craigslist. You were just read a code, twice, and then the other end automatically hung up.
ive been unable to get Craigslist's phone-token thing to call my VOIP DID without throwig errors. title there and here is misleading, i need a username/pasword to login to both VOIP and EMail accounts to fiddle with all of CL's post-approval stuff. it feels positively mid-90s
I haven't made any attempt to test it, but they probably block using VOIP and other non-fixed phone numbers too. There are APIs available to check whether a number is one of those types, so, say, you can't sign up for a bunch of numbers through Twilio and use those to spam Craigslist.