DTMF decoder

hello,
      I need to make a dtmf decoder using java. And it has to be decoded
using the pc sound card. Right now all i can do is connect 2 phones
through my modem.
      Any suggestions on how do i map the modem sound onto my pc sound
card?
My modem does not have a connection for a jumper. So i need to know how
can i do it without a jumper.

      Secondly.. Which algorihtm is preferable? Using FFT or Goertzel
algorithm? 
      im totally new to DSP. So i waould appreciate any detailed help.