Trice,
Your dilemma points to the idiocy of Porsche's approach. You would need to start a new service with Cingular or T-Mobile and have another number. And even if you already have GSM service from one of these carriers you would still need a second SIM card and a second number, since evidently GSM carriers will not provide cloned cards (either in the USA or Europe).
The SIM card cannot be physically transfered back and forth on a regular basis practically speaking.
My suggestion would be: instead of paying PAG nearly $1,000 for a truly half-arsed, out of date phone solution, spend a fraction of that on a new phone with Bluetooth and a cordless Bluetooth earpiece.
It is not of course a perfect solution - there is none with a Porsche for now - but it will allow you to keep your existing service and keep one number.
I assume Porsche will also offer a Bluetooth module once its Telematics vendor gets its act together. Otherwise they may soon be unique among premium car manufacturers in not offering the only logical, modern phone integration solution. Besides they will be able to charge another $960 or maybe $1,960 for something that comes standard on many $30,000 cars already!
FYI, this blog covers the subject:
http://bluetoothcar.typepad.com/main/PS: I'm just getting into this 'Porsche thing'. The car is a hoot to drive and looks absolutely wonderful, but their 'nickel and diming' on features is a joke. I had to take mine pre-ordered and the dealership got nearly everything right (at a price), but excluded the steering wheel audio control option - absolutely standard today on many $20,000 cars but a $960 option on a Porsche.
Since the engine drowns out the stereo as it revs up I'm constantly reaching over to adjust the volume and since I like to replay CD tracks I am constantly pressing the tiny, cheap little buttons on the console which is dangerously distracting and I wouldn't be surprised if they give out after a few months.
It must be incredibly inefficient for Porsche to design, manufacture and inventory all these different options. I'm sure they could offer their cars for a lot less and make even more profit if they just included everything, at least on a telematic level, standard.