Deleted messages
How do I retrieve deleted messages on my iPhone
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How do I retrieve deleted messages on my iPhone
If you're using iCloud Syncing for messages, once you delete the messages, they are gone.
If you're not using iCloud Syncing and you have a back up that contains the messages, you can restore your phone from that back up:
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Note that doing so will overwrite everything currently on the phone.
Deleted messages