I can use the iPhone 5 to place call via bluetooth in my car (Toyota Prius). However, when receiving a call via bluetooth, after pressing "answer" button, the call is connected, but no neither I or the other party can hear each other via the car's bluetooth. There are a number of people having the same issue. The car's bluetooth system is fine with iPhone 4 or 4S.
See people's complaints here:
Updating to iOS 6.1 fixed the problem for me.