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:
If your device doesn't ring when you receive notifications from an app, it is possible that its notification sounds are disabled. In order to enable them, you should go in the app's notification settings.
If your Wi-Fi or Mobile data is disabled, you won't be able to receive calls through apps such as Skype and Viber.
Check out the don't know how to enable/disable Wi-Fi or don't know how to enable/disable Mobile/Cellular data guides in order to learn how.
If you don’t receive the email in the next 2 minutes please check your junk folder, and add [email protected] to your safe sender list.