The smart money suggests that the headset plug and the cell phone jack are
both the three-connector types; think about it, you've got a common ground,
one hot lead for the earpiece and one hot lead for the microphone.  I think
Brent's right; you'll have to make a stereo y-cord, then just determine
which of the mono jacks is carrying the speaker output.  Now, you might not
get great results due to the probable impedance mismatch, but you should get
some audio.

John McCann