Keep Google Maps on top, make calls in background

[CraigVMN]

I have a LG ThinQ G7 which I use for GPS navigation via Google Maps. The phone connects through my cars bluetooth, and the calls play over the stereo. I have been searching for a way for incoming calls (or outgoing I make with the cars "phonebook") to take place in the background, leaving Maps on the screen, and navigation still taking place.

How can this be achieved?