No root needed, OnePlus 3 phone records calls perfectly in two directions, just install Boldbeast Recorder and go.
Please use these settings to record calls:
- Record Mode: 1 or 5
- Tune Audio Effect: No
- Tune Audio Route: Group3
Tested by OnePlus 3 A3000, A3003 Android 6.0, Android 7.0, Android 7.1.
In some recordings the caller's voice is weak?
Please see the  part for the reasons and the solutions in this page http://www.boldbeast.com/android/call_r … oting.html