Just buy a pay as you go sim card.
Get one that comes mounted in the plastic credit card, that you normally push out to insert into your phone.
Don't push out the SIM though, just slide the whole lot into the armrest slot, they are all made to a standard so the SIM is mounted in the right place.
The built in phone is very good, strong signal strength, txt messages etc.
Just divert your phone to it when you are driving it, and record your answer machine message as 'This is Dave's Maserati, i'm currently not been driven at the moment, but please leave a message for when Dave is back in the driving seat, thankyou!"