Why is it that when I use the phone screen mirroring function and then make a phone call, the sound does not come out of the head unit?

When using the Mirror Touch phone mirroring function (that is, mirroring the phone screen over a wired connection), if an incoming call is answered, the ringtone and call audio will indeed be output from the phone, and cannot be output through the car audio system. This is because the MirrorLink protocol forces hands-free mode to be enabled during a call, and since Mirror Touch connects to the head unit through the CarPlay protocol, the head unit disables its own Bluetooth phone audio channel, so call audio can only be output from the phone's speaker.

On the other hand, if you do not use the mirroring function, but instead connect the iPhone to Mirror Touch using wireless CarPlay (only supported by iPhones that support wireless CarPlay), call audio will be output normally through the car audio system, because wireless CarPlay itself establishes a complete audio channel, including call, navigation, and media audio.