What happens for me is when I hold the home button for siri, my car sees it as a phone call, so it'll activate the car's built-in mic, and the car display shows it as a phone call from myself.
My issue is that after I give siri a command, for example to send a text, she does so, then waits for another command. While she is waiting, my car will stay in phone call mode. As soon as a I press to home button to close siri and go to home screen, the call ends and everything back to normal. I wish Siri would close after she is done doing what I ask instead of waiting on me to either hit the speaker button for another command or the home button to close.
My car doesn't support accessing Siri in any way via the car controls. I have to manually do it from the phone.