Combine ring volume and notification volume

With a dumbphone, i don't want the volume rocker buttons to be too complicated. I want a simple way to make the phone more or less loud, depending on my environment. I think other phones do this nicely by offering a checkbox, "link ringer and notification volumes". So pressing the button will adjust both volumes.

The problem right now: when i reduce the call volume using the side buttons, the notifications are still loud. Or at least i think they will be.