How-to: Craigslist phone verification type system

Check out http://twilio.com for making voice calls programmatically.

e.g. http://www.twilio.com/docs/demos/appointmentreminder

Note this thread:

http://getsatisfaction.com/twilio/topics/are_there_any_futur...

... says they are only evaluating SMS at this time but an SMS gateway would be much easier to find out there (someone in the thread even pitches an option, http://www.textmarks.com )

Twilio rocks. I just got it going with Python from our chat system at work... took about 20 minutes.

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.

Looks like textmarks requires the person to subscribe before you can send messages to them: "To keep everybody happy and prevent abuse, you can only send texts to subscribers of your TextMark."

http://www.textmarks.com/dev/docs/send/?ref=devsb

sms option to consider: http://zeepmobile.com