Why does my iPhone play the Apex ringtone 4 times randomly but nothing is on the screen?

The screen stays black, it happens even in silent mode, Airplane mode and Do Not Disturb. I have no Application notifications that are set to Sound alerts with no banners or badges. It happens every couple of months. It usually happens about 5 times over an hour or so and then not again for a couple of months. The power button does not silence the sound. Swithing the phone off silent and back to silent while it's happening does not silence the sound. There is no way to stop it except for powering off the device completely, but when I turn it back on again later, it usually happens again once or twice. It is not an alarm or timer. I have never used to Apex ringtone for any reason, and no sounds are set to that in my settings. It has been happening on and off for over a year (6-8 times a year), always at the most inconvenient time like a hospital or public transport of somewhere I should be quiet.