Four reasons I am choosing NOT to cut the landline cord
VoIPInsider: "101 Things You Can Do With Asterisk Contest"

Is Craigslist blocking VoIP, Prepaid phone numbers in anti-spam effort?

craigslistlogo.jpgIs Craigslist really blocking phone numbers from VoIP service providers or pre-paid cell phones as an anti-spam measure?

Last night over on the VoIPinsider blog, Cory Andrews wrote that Craigslist is apparently blocking VoIP or prepaid cellular numbers as part of their anti-spam measures. Now I'm a huge fan of Craigslist and we've sold lots of items (including, now, our house) via Craigslist. But we've also seen the spam out there and personally been contacted in response to one of our ads by a sleazy individual who was trying to scam us out of money. Techdirt, in fact, says that the battle has been lost and that the spammers are taking over Craigslist. While it wasn't that dreadful in the Vermont Craigslist area, there certainly was some spam and you can understand the folks there wanting to do all they can to block spammers.

But to block VoIP service providers? Just as increasingly large numbers of users move over to VoIP services?


It seems a rather draconian - and misguided - measure. As the VoIP Insider article states:

A few months back, Craiglist instituted a telephone verification process that places an automated outbound call to a user placing a classified ad in certain categories. The call delivers a unique code using text to speech, which is then used by the poster to authenticate the ad they are placing.

This is an effective measure for dealing with spam, and a great thing for legitimizing the Craigslist user experience….but not so great if you are a Craigslist user and you also happen to be a VoIP or prepaid cellular user.

The problem is that Craigslist is categorically blocking legitimate VoIP and Pre-paid cellular users from authenticating themselves.

While I've not encountered the phone verification process in any posting I've done to Craigslist (but have seen CAPTCHA images all the time), I can see how the process would be useful in combatting spammers. The article goes on:

Craig’s uses a 3rd party service, to screen out VoIP and Pre-paid cellular numbers, and will not deliver an automated verification call to a number that is determined to be such. (Since only SPAMMERS use VoIP and Pre-Paid Cellular!!!) What sophisticated algorithm does use to identify VoIP numbers, you ask? They check the DID number to see who owns the NPA NXX X number block, and if the DID number is owned by Level 3 Communications, they classify it as VoIP of course. Whizbang!


My immediate question was whether this is for all VoIP service providers. This forum thread would seem to indicate that "fixed lines", even fixed VoIP lines, would come up as okay. So phone numbers from VoIP services from telephone carriers or cable providers would probably be okay. So it may just be the phone numbers of VoIP service providers who are not tied to a fixed infrastructure (and who provide connectivity to so many of the innovative services out there today!).


There are, though, some fairly obvious problems with this approach to blocking phone calls:

  1. LOCAL NUMBER PORTABILITY - Here in North America, phone numbers are "portable" (to a degree) via "Local Number Portability" (LNP) between carriers. So a phone number that may come up as "fixed" may in fact go to a VoIP service (and possibly to a spammer) due to LNP. Now perhaps the third-party service used by Craigslist is doing LNP database lookups.

  2. FORWARDING SERVICES - There are plenty of services (including one identified in the VoIP Insider article) that will forward calls to another phone number. I could even do this easily with something like Asterisk running on my (fixed) home phone number that then forwarded the call out via SIP.

It seems to me that it would be relatively trivial for any serious spammer to obtain a "fixed" phone number that would defeat this blocking mechanism. Certainly this would block some of the less savvy spammers who are just trying to use disposable phone numbers... but in the meantime it may well block legitimate posters who happen to use telephone numbers from VoIP service providers.


So is Craigslist really blocking VoIP phone numbers? Garrett Smith (from VoIP Insider) indicated in an email that someone there was in touch with Craig and Craig indicated he was not personally aware of the blocking. Obviously, someone within the Craigslist organization has engaged this external company,, in their ongoing efforts to fight spam. The blocking seems to lie in there. What needs to happen now is some conversation with those folks to understand what exactly it is they are doing.

We'll have some conversation, in any event, about this issue on today's Squawk Box at 11am US Eastern Time. Feel free to join us if you would like (you need to login via Facebook).

It's an interesting question - in the era when people can obtain cheap (even free) "disposable" phone numbers, how do you balance providing access to legitimate users while blocking using those numbers as a way to spam or perform other malicious actions?

Technorati Tags: , , ,