Why Isn't My iPhone Ringing Or Making Alert Sounds?
If your iPhone has suddenly stopped ringing or playing notification sounds, the problem is likely one of two things. From the Homescreen, drag up from the bottom of the screen to reveal the Control Center. You can see the Control Center in the image below:
First, make sure your volume is turned up. That's the bottom arrow in the picture above. Then make sure you haven't accidentally enabled "Do Not Disturb" mode. If moon-shaped icon is filled in (see the top red arrow above), then Do Not Disturb is turned on and your phone won't ring or make other sounds. Tap on the icon so it's no longer white to disable Do Not Disturb. Also double-check to make sure Airplane mode is not turned on (the icon to the far left of the Do Not Disturb icon).