Alarm sound volume

I recently set up a ring system for my grandparent’s. My grandfather is worried my grandmother might walk out of the house in the middle of the night or when he is outside.The problem we are having is that you can’t change the alarm volume that goes off on his iphone when a door sensor is triggered. So he wouldn’t hear it when he is sleeping or outside with a lawnmower going for example. There really needs to be a way to increase it on his phone or even a separate, portable alarm like the base station that he can carry with him that makes sounds like the base station.