Car phone??

As I was on the way home on the train tonight I got a call. Thanks to OS 3.0 I was informed that it was Belinda or Yip Law and they were calling from the car.
Turns out that it was my Dad and indeed he was calling from the car, parked near the train station, but how did my iPhone know that much??

I have my Google contacts synced to my iPhone via Nuevasync. Somewhere along the line it had registered both my Mum and my Dad with the same mobile number, except that it had registered Mum as having that number as car, so it took that as the preferential location and wha-la!! my parents seem to have a functioning car phone….