car radio overrides my bluetooth choice on iPhone

Hi. I use bluetooth connection for handsfree communication in my car. I also connect a bluetooth headset for use when outside my car. What happens is that when I call, or recieve a call, I choose by which means I want to speak. I touch my ear to use headset, I pick up my phone and answer directly or touch car radio. However, the radio is dominant and overrides me if I choose to call, or respond, directly or by headset. The moment I get connected with the other caller, the radio takes over and I have to manually open speaker options an choose again. Very annoying!

Deleting the radio bluetooth connection is not an option, as I also stream music and podcasts.

Does anyone know how to resolve this? My software is updated and I have tested various devices, both v.4 and v.5 always with the same result.

Looking forward to any respons.