if you have a Yosemite mac, and enable SMS forwarding, the SMS and iMessage will automatically go in the Messages app in the Mac. you can therefore archive these messages in the app. And even if the Mac crashes/dies, the Time machine backup will have a copy of the SMS+iMessages, forever.
Also, The SMS and iMessage are automatically saved in the iTunes/iCloud Backup, however, this has 2 drawbacks:
- To learn how to download text messages from iPhone to Mac via iCloud, follow these steps: 1. Go to your iPhone’s Settings > Messages and turn on the option of “Messages on iCloud”.
- Want to text someone on PC or Mac, instead of having to use your phone? You can now send and receive Android text messages on a desktop or laptop computer using Google's new Messages for web.
- If the iPhone is lost/dies, you'll be able to recover the messages only by putting the backup on a newly reset/bought iPhone.
- the backup only saves current SMS/iMessage, which means that if you manually delete from the iPhone the messages, it will also delete from the backup next time the backup syncs. This therefore doesn't allow 'archiving'.
Send text messages via your desktop computer using TextMagic Messenger – a two-way bulk SMS software application. Bulk SMS Software Send bulk SMS directly from your PC computer or Mac. Tick all messages you want to view on PC, and hit 'Recover' button to save them on your computer. After that, directly open the output file to read the iPhone messages freely. 2) Get & View iPhone Text Messages from iCloud Backups. Select 'Recover from iCloud Backup File', then login with your iCloud account according to the onscreen instructions.
So the first solution is best if you have a Mac, especially since it's for legal reasons, for which it is recommended to have 2 versions of the files.
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Hope this helps 🙂
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<Edit>: Also, you'll probably never find an app that syncs messages, since iOS is a sandbox OS, meaning the Apps don't have access to the data of other apps. so Messages data isn't available to any other app.