I have Marvels (M70-R’s) and the phone (Android Galaxy Note 9) switches over the the car Bluetooth when I start up the car to play music and phone calls through the car’s audio system.
Once I’m in a call, the Android call progress screen has an audio source selector so I can manually switch back to the HA’s if I want to or manually choose handset or speakerphone also.
The intelligence (in the case of the Android) is in the phone, and it makes the decision that when you’re in the car, the car audio takes precedence. Since the phone is multi-connectable, it maintains the connection with the HA’s to make it easy to switch back to them.
Notably, when the phone rings, it rings in the phone, through the car audio, AND through the HA’s. I might even be able to answer a call on the Marvels by pressing the volume rocker and override the phone’s default to the car audio. I’ll let you know if that’s the case after I try it.