If you synced your phone with these deleted messages on it you can restore your phone from the backup to recover you messages. This will revert all your settings and data to those contained in the backup. To do this, first import any recent photos to your computer or they will be lost. Then, prior to connecting your phone, open iTunes and go to Preferences. On the Devices tab check "Prevent...from syncing automatically." Now connect your phone and when it appears in iTunes on the left sidebar right-click on its name and choose Transfer Purchases (to transfer any recent purchases to your iTunes library). Then right-click again and choose Restore from Backup. When complete you can go back and restore automatic syncing if you prefer this.