I have the same issue, fortunately the fix easy. Unfortunately the fix is to buy a bluetooth headset. Alternatively use a Starbucks cup in the cup holder with attached piece of string to gain almost the same quality as the Porsche option!
I'm looking into a good bluetooth handset for my Nokia. A while ago I learned that the PCM telephone module does not have both of the US GSM frequencies. Its a tri-band unit and not a quad-band.
Well known problem. Porsche is working on it. Apparently the mic is affixed on the steering stalk. Some people move this up near the visor or something, your dealer should know. It is really disgusting that on such a car we get a useless phone, no BT and no iPod connection.
Dan - any sort of speaker phone in a cab is problematic! Top up or down, the ambient noise is just to great to have a clear conversation. The best result I've had is with a Etymotic ear phone, which actually sits inside the ear canal.