Making a call Google Assistant by saying "Call Mom & Dad" shows "App is not installed."
I thought I'd come back and post an update / conclusion to this question.
With the help of others, it was determined that the Android Telecom server on my Android Head Unit was missing code to handle the voice call.  After replacing it and then adding a custom phone dialer modified to pass the call information to the GVC CallHandler, which then passes the information to the Bluetooth attached phone, the voice calling worked.
Details can be found on the XDA forums under the General / Android Auto / Android Head Units forum in the thread named &quot;Modding your Joying/FYT SC9853i unit without root&quot;, page 31.
Note: I thought originally that this was a common problem.  However, this is not a general problem with Android but a specific problem with my head unit and possibly other FYT head units based on the Intel sc9853i CPU running Android 8.1   This may also apply to other units, but you'll have to go through and determine that on your own.