Schwinn, we recently had a need to do just what you are trying to do. Roomie's mom left the cutest Easter message on our voice mail. Fortunately, our playback device just plays it, no headset needed. We used a microphone connected to our XP machine, aimed it at the speaker and used the built in sound recorder to tape it. It saved it as a .wav file.
I am not sure how or if a headset will work, but maybe an external microphone aimed at the ear part of the headset???
If you have XP or Vista, both have Sound Recorder - real easy to use.
Let me know if that works or if I can help with the sound recorder part.
A dog is the only thing on earth that loves you more than he loves himself.