can deleted text messages be retrieved?

You would have to restore your phone from your icloud or itunes backup, if those message were included in the backup.

http://support.apple.com/kb/HT1766

entirely depends on when you deleted them. In case you had synced your iphone with itunes anytime in the past, then you can definitely recover them all. There are a couple of backup extraction softwares available.. and I would personally recommend Text Saver for the price and ease of use. I got back over 3K messages which were lost when I iphone got bricked during a software update

You can download it from www.myphonedata.com/iphone