iPhone disable calling on one device

[Timothy Bucktu]

Let's say you have two Bluetooth devices, both have microphones and so are capable of being used for calls. In my case, I have a car stereo which is Bluetooth calling/phone capable, and an Alexa Auto. I want to stop Alexa Auto being a device capable of calling, and set the car stereo as the default. At the moment, if a call comes through, Alexa flashes but I can't hear anything unless I switch to Aux input. The car stereo doesn't even bother ringing when Alexa is around.
On Android its easy...just click the cog next to the device, and turn off 'calls'.
Any ideas appreciated.