How to output Android head unit Bluetooth call sound to USB DAC?

I am trying to use a USB DAC with my Android head unit. While it works well with most apps, it doesn’t output sound when making Bluetooth calls. I did some research, and it’s said that because the mic is used during the calls, the system handles the sound differently. It mentions that there should be settings to force the Bluetooth call sound to be output through USB. However, I couldn’t find the settings and also couldn’t use apps mentioned like Lesser AudioSwitch, for they are not supported in Android 9 and later. Anyone can help shed some light on the matter? Much appreciated.