As of Oct 2016 anyhow, this is a throwaway VOIP number being used from scamming. They will probably burn this number in a few weeks. They are entering your number into Craigslist when creating an account, and then they will text you from this number (or another throwaway) pretending that they represent Craigslist and need the code. Don't even talk to them. It's probably a bot anyhow, and definitely a scam. They're trying to use your number for verification because they're blocked as spammers.
The robot that calls you from 415-610-5110 is a legitimate Craigslist autodialer. The person who follows up asking for the code you just got is a scammer.
For you amusement, the message I got said:
"Hey, There How Are you?
I am Linda From Craigslist I Already send the secret code
if you real check your phone messages
give me the code I will call you"
(All capitalization, spelling, and grammatical errors intact)