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If Messages in iCloud is enabled, the messages cannot be retrieved. If you're unsure if this feature is turned on, here's how to check: Use Messages in iCloud. If Messages in iCloud isn't turned on, and you have a backup that contains the messages, they can be restored with these steps: Restore your iPhone, iPad, or iPod touch from a backup. Keep in mind, any data that has been put onto your iPhone after the backup was created will be lost. If you have made a backup of the iPhone since you deleted the message, you will not be able to retrieve it either.

The trick is that you must have backed up your iPhone while the messages were still on the phone.....and.....you must not have backed up again. Otherwise, the next backup would pick up the fact that the messages were deleted and back up that way.

IF......you are backing up the iPhone to a Mac......AND......the Mac is backing up to another hard drive using Time Machine........THEN.......you might be able to go back through the history of backups on Time Machine and restore the backup with the messages. Unfortunately, you will lose all the data that you have added to the iPhone since the date of the backup.