Fix Unchangeable Ringer Volume Issue in iPhone X

When you bought your new iPhone X, inserted a SIM card in it, and tried calling the number from a different phone, did you notice that the ringer volume remained constant and unchangeable even if you moved the volume slider up to the maximum level?
If so, be assured that this behavior is not due to any issue in your iPhone, but because Attention Aware Features are enabled and working correctly. In other words, by default this is how your iPhone X should work.
What are Attention Aware Features?
Attention Aware Features rely on TrueDepth camera that iPhone X has. The TrueDepth camera recognizes if you’re looking at your iPhone’s screen, and makes the Attention Aware Features respond accordingly.
The Attention Aware Features option, when enabled, ensures that as long as you’re looking at your iPhone’s screen:
[listNum]■The display brightness is not dimmed.
■The ringer volume is not too loud.
Can You Turn Off Attention Aware Features?
Yes you can. Both the Attention Aware Features can be turned off by disabling the Attention Aware Features option from within the Face ID & Passcode window. Here’s how:
    Get to the Settings window
    Tap Settings from the Home screen. This opens the Settings window that allows you to configure and manage your iPhone X’s settings.

    Get to the Face ID & Passcode window
    Tap Face ID & Passcode. This opens the Face ID & Passcode window. This window enables you to set up and manage your Face ID, passcode, and Attention Aware Features.

    Turn off Attention Aware Features

Tap the Attention Aware Features toggle button. This turns off both the Attention Aware Features discussed above.

Note: You can re-enable both the Attention Aware Features by tapping the Attention Aware Features toggle button again.