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Disable the airplane mode
Maybe your device doesn't ring because of the Airplane mode. If you don't see the signal strength, nor the operator's name in the upper-left corner of the screen, but you see a small airplane icon, the airplane mode is enabled.
If your iPhone doesn't ring, but you receive notifications in your voicemail that somebody called you, it's possible that you have enabled the Do Not Disturb option. If you see a half-moon icon next to the battery icon, this option is enabled.
You can disable this option via the Action Center or via Settings:
Via the Action Center:
Swipe up from the lower part of the screen and tap the half-moon icon.
Go to Settings -> Tap Do Not Disturb -> Move the switch next to Manual to the left.
It is possible that the ringtone you set to notify you when someone is calling you or when you receive notifications of any kind is deleted or moved to a different location. Try setting another ringtone in the device's settings.
It is very important to update the device's operating system on time as the updates contain fixes to various bugs. If you don't receive a notification about the update, check if one is available manually.