The iPhone comes with various sound customization options when receiving calls, text messages or other notifications, all falling in one of two notification settings: Ring or Silent. For instance, you can enable vibration for Silent or Ring mode, allowing you to get an extra level of notification in case you cannot hear your phone. Enable vibration using the iPhone's built-in sound customization options.
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Choose "Settings" from the iPhone's home screen.
Slide the "On/Off" switch to "On" next to Vibrate in the Silent section.
Slide the "On/Off" switch to "On" next to Vibrate in the Ring section.