This has happened 2-3 times for me. Each time it has happened while on a phone call with someone and when I have been messing around with input sources, either on my phone or on the car. For example, when switching around from different bluetooth devices (if I happen to be using bluetooth headphones in the car), USB input or bluetooth input on car, etc. This switching around from output/input sources seems to cause the stereo system to become confused. At this point the stereo system completely locks up and does nothing. You can no longer hear the person on the phone and you have to switch the phone to speaker to continue the call. You can't change to the radio, you can't change settings, pressing the stereo power button (volume knob) does nothing, and even after hanging up the call, the phone call screen is still displayed on the stereo system.

The only way I have been able to fix this is to pull over and turn the car off and back on. This is extremely annoying, especially if you're on a long stretch of highway with a while before you can get to the next exit because then you're stuck riding in silence. For reference I have an iPhone 8 that is completely up to date OS. It would be nice if there was at least a way to perform a hard stereo system reset while driving, perhaps by holding down a couple buttons at the same time or something... Of course it'd be even better if this never happened at all.