Apple iPhone OS 3.1 - Manual (Page 90)

Managing Previews and Alerts
By default, iPhone displays a preview of new messages when iPhone is locked or you
are using another application. You can turn this preview on or off in Settings. You can
also enable alerts for text messages.
Turn previews on or off: In Settings, choose Messages and tap Show Preview.
Repeat previews: In Settings, choose Messages and tap Repeat Alert. If you don’t
respond to the first preview of a new message, the preview will be displayed twice
more.
Set whether an alert sounds when you get a text message or preview: In Settings,
choose Sounds, then tap New Text Message. Tap the alert sound you want, or None if
you don’t want an audible alert.
Important: If the Ring/Silent switch is off, text alerts won’t sound.
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