While the iPhone's silent mode is useful in curbing public annoyance, you might not feel or hear the phone vibrate if it's sitting on a nearby desk or table. A built-in feature can combat the problem by causing the camera flash to fire whenever the phone rings. This can alert you to new calls and messages from across the room but only when your iPhone is locked.
Accessibility Feature
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The LED Flash for Alerts feature is primarily intended for iPhone users with hearing impairments. As these users may be physically unable to hear audible alerts or the buzzing of their iPhone's vibration, Apple designed the feature to rapidly toggle the camera flash on and off instead. Assuming the phone is placed face down, hearing-impaired users can easily tell when the phone is going off.
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Enabling Flash Alerts
You can turn Flash for Alerts on by tapping the "Settings" icon from the home screen and selecting "General" from the list of options. Tap "Accessibility" and scroll down to "LED Flash for Alerts." Slide the toggle switch to the "ON" position and your iPhone will then flash whenever a call or message is received. You may wish to call your iPhone from another device to ensure that the flash feature is working correctly.
Battery Impact
While the flash alert feature may be handy, it will also cause your battery to drain more quickly. If you receive many calls and texts per day, you'll see a difference in your iPhone's daily battery life. Plan ahead by carrying a charger or charging cable around with you if necessary.
Public Consideration
You may wish to consider the Flash for Alerts feature's potential impact on those around you. If your phone is on the table during a business meeting or intimate dinner for two, the sudden flashing could be distracting to others. If you keep your phone in a cup holder at movie theaters, the flashing could be even more disruptive. You may wish to selectively disable this feature when not absolutely necessary.
Software Disclaimer
Depending on your version of iOS -- the operating system that runs the iPhone -- the steps outlined above may be slightly different.