Make it work perfectly on Pi (w/root)
On Pixel Experience ROM even with root I could only make the app record via the mic on any source I tried (symptoms: other side sounds weaker, muting in the dialer during call also mutes the recording, no sound via bluetooth etc.) UNTIL I found and followed this guide here: https://forum.xda-developers.com/mi-6/themes/call-recording-android-9-pie-roms-root-t3940614
IMHO this should be posted as a wiki as I see many people asking about it.
Edit: Alex Kiraly beat me to it, however I think using the Systemizer module is easier and less technical.
Edited by AG