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).