Sorry, not an iPhone user but that doesn't seem right to me. I have an Android device and rarely plug-in via USB and I'm able to make calls, receive/send texts, etc without being plugged in.
I know there is typically a setting for bluetooth audio vs bluetooth audio-and-phone, so perhaps you only have the prior enabled. If you don't recall making a selection to only enable audio then sometimes this occurs when the bluetooth protocol doesn't fully sync with the vehicle entirely when connected. You might need to disable bluetooth on your phone and then enable it again. That usually fixes it for me (most of my experience was with my last vehicle but bluetooth problems can occur with any vehicle).
Hopefully that helps, someone with an iPhone chimes in or better yet, you decide to upgrade to an Android
I know there is typically a setting for bluetooth audio vs bluetooth audio-and-phone, so perhaps you only have the prior enabled. If you don't recall making a selection to only enable audio then sometimes this occurs when the bluetooth protocol doesn't fully sync with the vehicle entirely when connected. You might need to disable bluetooth on your phone and then enable it again. That usually fixes it for me (most of my experience was with my last vehicle but bluetooth problems can occur with any vehicle).
Hopefully that helps, someone with an iPhone chimes in or better yet, you decide to upgrade to an Android