How would I go about building a craigslist-type phone verification system, which, when given a phone number, either texts or calls the number with a code that the user can use to verify that it's their phone?
Check out <a href="http://twilio.com" rel="nofollow">http://twilio.com</a> for making voice calls programmatically.<p>e.g. <a href="http://www.twilio.com/docs/demos/appointmentreminder" rel="nofollow">http://www.twilio.com/docs/demos/appointmentreminder</a>
Using an SMS aggregator such as CellTrust, you could send the user a code through a shared short code. Or, if you're willing to put up with short ads at the bottom of messages, you could try TextMarks.