TO enable this Changes,
Head first, to your iPhone's "Settings" menu, then scroll down to select the "General" option.
From the "General" menu, tab, head over to "Accessibility" options.
Then, scroll down and look for the section entitled "Hearing." The LED flash setting is part of Apple's suite of tools to help the hearing impaired.
Now you need to select the "LED Flash for Alerts" to on, so you see a green oblong under the virtual button. Also, when your iPhone's screen is locked and you get any alert including SMS, MMS or Call, your Apple iPhone's flash will go off several times to alert you.
HOW to Turn OFF iPhone Flash When You Get a Text
To turn this amazing functionality off, you need to toggle the "LED Flash for Alerts" option back to off, from step 3 above.