I know this is an older post but I have an update for anyone who is interested.
We have recently started a project at Automic Software to use our Automation Engine to trigger phone calls when an urgent case is created after hours. So far, our demo has been a success in that we are using the Automation Engine to call users. We are using the Twilio API to make the phone calls which is around $0.015 per call:
https://www.twilio.com/Twilio has a very sophisticated and robust API which allows the execution of XML or PHP to determine how Twilio handles the call. In our case, we simply have the Automation Engine executing our RA REST Web Services job which then calls the Twilio API and tells it what number to dial and what the automated message should say. If anyone is interested further please let me know. I'll will be writing this up in a bit more detail within the upcoming weeks as well.
Hope this helps!