linux telephony?

[Krzys Majewski]

I've  got a  modem,  a  soundcard, some  speakers,  and potentially  a
microphone. Does this mean I can use my linux box as a telephone? 
This would be particularly useful  for things like "the secretary says
he's  on another  line can  you hold  for a  long long  time  while we
entertain  you with  the latest  kenny g  offering", since  my current
telephone does not have a hands-free/speakerphone option. 
-chris