My iPhone X is Not Sounding a Text Alert: WHY??

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Can you tell me why my new iPhone X is not sounding a text alert, a call alert? 

 Yes this is my first iPhone, so this may be the cause of not knowing what was up with not hearing the alerts to both texts and calls, so forgive my unfamiliarity. So this is perhaps for any of you who might also be new first time owners of an iPhone X, and maybe for other models too. It turns out there was a slider on the left side of the phone but hidden under the cover. And without even looking I pulled off the Defender rubber cover to see a small switch/slider that was showing a very thin line of red. Once I slid it to black I could hear the alerts. Just for anyone not knowing here is the slider switch, borrowed from a photo online (from of all places-androidcentral).

 

And here's the link to how to find it and slide it at macreports.com