I know that there are some projects to turn Linux into a answering machine.
Basicly you need a modem that can handle voice. (preferabley a hardware modem since winmodems need special drivers that makes using them in Linux a pain).
Then you have these things called "getty"s. They are basicly console login programs (the command line stuff outside the X windows GUI). But there have been variations to make it so you can use sound instead of getting a login screen. Basicly telephone tones. You know "Hit 1 for blah blah blah, hit 2 for yadda yadda yadda" etc.
So these projects turn it into a automated answering service. Be able to do stuff like e-mail yourself mp3 versions of messages people leave for you on your computer. Don't know much beyond that, but I wouldn't think it would be too difficult to use your computer as a phone.
Normal computer geeks just tend to "build their own" setup using basic tools like vgetty (voice version of (m)getty) but
here is a program that is already setup to use it. Of course with anything this geeky you need to have some knowledge of Linux.
Hell VoIP is just a step beyond that.
I am sure that they are plenty of commercial software apps to do that with Windows (I am a linux type guy). I would look for software bundled with various voice-capable modems, I remember vagely seeing boxes of modems with little smiling people with headsets on them.