Apple iPhone (For iOS 6.1) - Manual (Page 66)

Chapter 9 Messages 66
Messages settings
Go to Settings > Messages to set options for Messages, including:
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Turning iMessage on or off
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Notifying others when you’ve read their messages
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Specifying an Apple ID or email address to use with Messages
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SMS and MMS options
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Showing the Subject field
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Showing the character count
Manage notifications for messages: See Do Not Disturb and Notifications on page 132.
Set the alert sound for incoming text messages: See Sounds on page 139.