This fix works, I was having the same issue...
I have a USB PC Phone with headset attached to the PC and wanted to use it for my cell calls while working on the PC. For the USB PC Phone mic & audio to behave (during cell calls) I had to go into the Android cell phone Apps under Settings (in the Link to Windows app) and under Permissions add the Microphone (shown below). You would think this would be preconfigured but for privacy reasons the phone's owner must add this.
Once this was accomplished the USB PC Phone mic worked with the cell phone through the PC link, however I could only control where the output audio came from by changing the default Windows audio out settings (shown below). Just annoying that I can't assign this under the Phone Link app settings in Windows for the preference during calls.
This was much simpler under the Skype setup, Phone Link should have at a minimum the same telephony configuration.
Hope this helps other...
Greg