It's an iPhone 7, running iOS 12.3.1.
Unfortunately I have no access to any other devices with a ringer switch, so can't test a different version of iOS.
(Also, as a side note, please don't take this as a criticism of AppGameKit, I am just surprised at this unusual behaviour. Overall I have been testing AppGameKit across my iPhone, iPad, Android phone, Mac and PC over the last few days and this is the only thing I have found that is of concern. AppGameKit seems fantastic and just what I'm after - it reminds me a lot of Blitz Basic, of which I spent many (hundreds of!) happy hours on both my Amiga and subsequently PC.)