Bluetooth audio works except for phone audio. Please help

[Jirokichi]

I need your wisdom. My Aqua M4's bluetooth works just fine but one exception - phone audio. In my car, I have a bluetooth receiver (Unity) which connects to an AUX jack. I can play music, Audible, and Google Map, etc. through BT. However, the phone's voice does not go through BT; it uses Aqua's speaker.  I have looked for any switch and so on in the Phone program.  In Google Map, it has a bluetooth audio switch.  In the Phone setting, there is nothing like that. So, please help.

[Uliwooly]

When you are on a call do you get the option to switch to the Bluetooth device? It should be on the loudspeaker option

[Jirokichi]

Thank you.  No, there is no switch on the phone (Aqua M4). Roverbeat Unify is a BT receiver (4.0), which is connected to the AUX jack to my radio. Unity does not have any user interface except for an On/OFF button. This is what happens.  M4 is connected to other apps, such as GPS and Audible, etc. I am streaming these perfectly, and then I get a call. The ring cannot be heard; the screen is saying that I am receiving a call, so I answer it. I need to press "Audio Output" on the Phone interface to hear. This is basically the same when I am not using any other BT apps.  Simply, only the Phone does not give any Audio.  For your information, I have a iPhone 6S as well, and this, either, shows the same issue. I am now thinking that somehow Roverbeat Unity itself does not work with any smartphone Audio at all.  Possible? Anyway, if that is the case, it is not an issue of M4.  Does somebody have similar issues?