Appreciate the reply, but no, in my case it has nothing to do with holding anything on the phone. When I'm in my car, my phone sits in its holder that is attached to my dashboard. Any time I either simply push the little microphone icon on any screen on my phone where a keyboard appears (like texting, a search, navigation, whatever), AS WELL AS any time I simply say " hey Siri" and not even touch the phone, once the microphone or Siri is activated, the car calls my own number. The car display shows "active call" and continues to show the seconds counting of how long it's been on an active call. Any audio that had been playing stops just as it does when you're on a phone call. So I speak the words of what I want to say or what I'm asking Siri. Once I'm done speaking and/or I touch the microphone icon again to finish my voice-to-text task, then the call hangs up, the car makes the "hang up" sound, the car display stops showing active call, and the previous audio begins playing again.
Tonight I discovered only two instances where this would NOT happen, and that is by using voice-to-text on a Google feature, such as Google search, or within Google maps. I was shocked! But somehow those Google apps don't have the same glitch. I could actually just touch the microphone, the screen on the phone changes to something like a microphone (can't remember exactly) or "listening" or something... anyway I can actually speak out what I'm trying to say. And then when I'm done, the phone performs what I told it to do or write in the words I said. But anything else and the car calls itself.
I have spoken to multiple Apple professionals, even "senior advisers" and no one has any answer other than they've heard of it before. They think it's because my car does not have Apple's Car Play software. It's a 2017 Mazda, which apparently is a year or two behind when Mazda started equipping cars with Apple's software.
Long story I know, sorry... but it's just so frustrating, along with a handful of other functions that iPhone just flat out cannot do that the Android with ease. So many features that I thought were no brainers that the iPhone does not have. :(