I don't think I have ever used that button, I always use the Voice Call button and go through Siri to initiate a call.
Does the phone button bring up anything on the screen when you press it? (I don't have my car with me at the moment).
Yeah, my other car allows you to re-dial or cycle through some other options. I just wanted to know what it does. If that's all it does then I'm cool with it. I just thought it was broken... lol