Bluetooth call sound very loud (in car)

[_Beni_]

I know there already is a thread about cars, but it isn't that similar to mine.
I'm using the latest OxygenOS with Boeffla-Kernel and I'm connected to a Philips CE235BT car radio.
The call sound if i receive a call is really loud and i didn't find any option to change it.
Is there any app out there, which can regulate the bluetooth call volume only? Or can it be changed by editing some system files?

The playing sound of bluetooth is OK, the AudioTuner is also disabled.

 

[_Beni_]

It seems I just needed to reduce the volume while calling, it was at the maximum level. The level is stored and if i end the call it goes back to the music level.