It's definetly possible, especially these days with voip (voice over IP), it's very easy to have a computer make automated calls. You'll probably need a voip-provider that will take your datastreams and route them into the "real" phone network. I haven't done the client side, I've only written "answering" programs (those annoying "Please enter your code now ... I understood 1 2 3 4 - is that correct?"-things) based on asterisk. Those were very easy so I'm guessing the client side isn't much harder.