mute navigation when in a phonecall

I'm new to this forum, so perhaps my question is already answered.
if so, I'm sorry. I couldn't find the right thing, so I ask it here...

I own a Prius for about 4 weeks now.
I live in the Netherlands, so it's a European model and perhaps has some differences to the US models.

I'm very pleased with my Prius. It has everything I need.
But there's one thing that's bothering me.

I have the Prius Tech edition, that comes with navigation and a bluetooth carkit.
It all works fine and when the navigation system is guiding me, it uses the left speaker for instructions.
When I'm in a phonecall, the carkit also uses the left speaker.
When the carkit is operating, there are no voice instructions from the navigation system.
That's all fine.
When I get a phonecall, the screen switches to the telephone screen, so I can answer the call.
But when in a call, I don't need to look at the telephone screen all the time. Especially when I need directions.
So I switch to the navigation screen.
But when I switch to the navigation screen when talking on the phone, the system mutes my phone, repeates the last navigation instrunction and turnes the phone on again.
In this situation I loose some seconds of my call when I'm not able to hear the one I'm talking to.

Now I have to say to the other end: 'hold on please, I have to switch to another screen. I won't be able to hear you for some seconds...'

Is there a setting for this?
I tried all the settings on the phone and navigation, but couldn't find it.

thanks.