I've found the same problem however the workaround I've used is as follows: on the handset in google settings > voice > activate "Bluetooth headset - records audio through Bluetooth headset if available". Also activate 'OK Google detection' > Trusted Voice.
This will pick up "OK google" via the mic and make the call with audio via the head unit.
I don't really like this as a solution as I don't like using OK Google, I don't want google listening all the time. And using Trusted Voice is not very secure as it basically unlocks your handset. Google even state it's not very secure when you activate it.
I have the Google Assistant icon on the homepage of the head unit, I'd like to activate google assistant by touching the icon, then making the voice request. However it's trying to use the head unit to make the call so gives the message "not yet able to make calls".
Still looking for a solution but I don't think the answer lies with google assistant, as it's running on an android device and therefore will only try to use the device it's running on to make the call.
You can say "Ok google, open Bluetooth" (assuming the Bluetooth dialer on you head unit is called "Bluetooth") and google assistant will open the dialer app, as this is system level. However you can't then use google assistant to control the app, as it has no access to it and/or knowledge of it's function.