Radio Shack (and, I imagine, any electronics parts stores) used to have a little suctioncup attaching mic... went on the outside of the ear part of the handset.
If you could get ahold of one of those, you could probably line-in it to your Pbook, and pick up the signal using Peak or Ambrosia's WireTap. Then convert to whatever...
There's prolly a more elegant solution, though...