As far as I know, the phone dialer simply existed to allow a person to quickly dial a number from an address book or similar. A normal phone would still be needed for it.

I'm not really sure what software to recommend to do what you want, as my experience with it was with a Gateway specific application, and software that came with an old US Robotics voice modem. A search for PC answering machine software might turn up something, as many of those programs allow phone use.