Connect with WebIDE, go to the "Device Settings" and set the following:
1. callrecording.mode (string) = on or auto (manual or automatic recording);
2. callrecording.notification.enabled = false - just for sake of it;
3. callrecording.vibration.enabled = false - just for sake of it;
4. callrecording.file.format = wav (default is 3gpp, IIRC one can specify ogg and opus as well).
Now, if your KaiOS phone has a Camera button, recording will be toggled with it. If there's no Camera button, recording can be toggled by pressing Left arrow. A "rec" icon will appear.
The resulting files will be stored in the "callrecording" directory of your main storage (internal or sdcard).
P.S. I'm going to release a utility that allows to switch these settings soon.