cyril7: Hi, as others have mentioned give the old modem trick the flick, so last century, if you want to invest in learning stuff then get a 2talk or similar account, setup a soft phone (X-lite is available on nix aswell as Windoze) and get a USB headset. I use this very setup (on Windows and Linux) to work with some guys in Singapore on a project that wont go away :). Works a treat.
Obviously making calls to the network has a cost but to other PCs with VOIP connections its free.
He is right put Asterisk on that old PC if you want to learn something otherwise by a cordless phone for $30.
Give this a try its dead simple to configure.
Then sign up for a free 2talk account and you can even add a local number for $6.90 a month now you have 2 lines in the house a PBX and you learnt something.