Orange 'Single Number' service costs £3 a month and works seamlessly, you don't have to divert any calls as all calls go to your car phone when your mobile is switched off. Or, you specify this to default to the car phone and calls then go to your handset when the car is not in use. Downside of this second option is that if someone else (wife, girlfriend, garage etc) uses your car, they'll receive all your calls. And I had problems receiving texts.
My current C2S came without the phone module so I hade the Nav-TV (Tooki) bluetooth module fitted - brilliant! Interfaces seamlessly with the PCM, keypad etc and is much better, in my opinion, that Porsche's own phone module.
critch