I have a a 2009 VW GTI and I have noticed that (regardless of what the instructions say) you have to go to the bluetooth screen in the settings on the iPhone 4s, then stay at that screen while you search for and pair the phone via the controls on the car steering wheel through the DIS options. That seems to be the only way to pair the phone.
Just discouvered how to pair.
1. Go into Settings on your iphone.
2. Go to General.
3. Go to Bluetooth.
4. On the car go to the select phone.
5. The car system will search and find the phone. A 4 digit area will show up on your iphone.
6. Put in your unique bluetooth/phone pin. This is used when you first set up the phone on your
car. If you can't remember, just go back to the car's orginal pairing mode and it will give it to you.
7. Once you enter your pin, press "pair" and voila! It works for me for both iphones.
I've also turned off the car and restarted and the car automatically recognizes the iphone.
Hope this helps some people.