When I have my iPhone plugged in via usb, all Siri functions work great through the car. Don't need to push the voice command button on the steering wheel (which I agree, is useless-I would get the same message as you about command not being available). Just use Siri like you would at home. I can have it read and replay to texts, play music. make calls and all audio comes from the cars speakers. When you are finished, it goes back to the music that was paused. So far, it's worked great!I'm having the same issue with my iPhone. I have tried both bluetooth and USB connection. Always says command not available .
As an aside, I have noticed that the album art loads on the screen when the phone is plugged into USB.