To dial phones in most situations all you need is an app that will actually play the audio (DTMF) tones and hold the phone up to the speakers and it should work in most cases (some VoIP systems might try to filter out DTMF). I used to do this years ago with my powerbook at pay phones and freak people out LOL
If you can find an app that will play the DTMF tones for your contacts (or maybe there's an address book plugin) this might work for you.
This app seems to do the trick, although for my needs $25 is a bit hefty ...
Dialectic : The Scriptable Dialer for Mac!