Call recorder on devices that have at least dedicated volume keys
This trick only works on KaiOS 2.5.2 or higher (if it will not officially implemented):
Connect with WebIDE, go to the "Device Settings" and set the following:
callrecording.mode (string) = on or auto (manual or automatic recording);
callrecording.notification.enabled = false - just for sake of it;
callrecording.vibration.enabled = false - just for sake of it;
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).