Thanks for the posts guys. My car manual clearly implied that some network providers could do this, but there again it's a froggie car, so heaven knows what they can get away with there.
In no way is it important enough to warrant illegal acts just to satisfy my techie desires. I guess simply setting my Blackberry to forward any incoming calls to a PAYG SIM in the car's phone will suffice for while I'm driving.
I could always just carry on using my Bluetooth earpiece with the Blackberry anyway and ignore the onboard phone altogether. Seems such a waste though.
Dee747