I just installed a Pioneer FH-X720BT CD receiver in my vehicle. I did not install the external microphone that came with it as I didn't want a wire hanging around, and I assumed that the microphone on my phone would work. However, when my phone is connected via bluetooth, I can answer calls and hear the caller through the speakers, but they cannot hear me, almost as if the receiver is trying to use the microphone even though it's not hooked up. Must I hook up the external microphone that it came with to make phone calls? Or is there a setting on the phone or receiver I need to change? My phone is a OnePlus One (Android 4.4.4).
In this case, the phone is only used as a conduit or a means by which the stereo acts as the mouth (speakers) and ears (mic) through the bluetooth connection. You'll have to hook up the microphone in order for it to work, because that's what the stereo is expecting.
My suggestion is to find a spot which is innocuous and run the mic to that spot. It should be fairly easy to hide the wire through any of many different avenues in the dash or upholstery to do so.