Ok, so I took the LG "zero-install" hands-free kit out of my other vehicle and put it into the Boxster.
It's something like this:
http://www.wirelessgalaxy.com/ac/lg_vx3100_zero_install.html
It's "passable". but not altogether great (though in a quiet Lexus, it's fantastic).
It's a true hands-free unit as opposed to the the headset. It plugs into the cig lighter, has a speaker and a long cord to a small mike that you clip onto the visor.
I didn't try it with the top up since it was a beautiful day today. But with the top down it was only passable. The speaker got drowned up at higher speeds and at higher RPMs. It became a bit frustrating to use.
I may have to just let the phone ring and call people back when it's convenient.