As in automatically removing them from posts? That would be very stupid because some people use their Google Voice number exclusively, and it shouldn't be up to them to decide how I should be contacted.
It's very annoying, they do it to combat spammers but as an innocent user I get hit all the time with the phone verification.
Craigslist requires that your number be identified as "Fixed Line" or "Mobile" (not "Non-Fixed VoIP" or "Prepaid Mobile") by their provider, ReduceFraud.com, in order for you to be able to complete phone number verification and post in certain sections on CraigsList.