Yes, I too have contemplated phone verification since that is what Craigslist is now using but I would have to code my own plugin and it is not clear what that would entail.
Right now, I am thinking of a two step process. The first step is that the user registers with email confirmation an email address that is the same one that is being used on an authority site, e.g. Google merchants, Bing, a local Chamber of Commerce, etc. Once this is completed, I would have to manually verify that the email address is a legitimate by comparing it to one of the authority sites. Unfortunately, it is a manual process since here is no way for me to automate this.
The purpose is to minimize fraudulent or prank ads.
Rich