the texts are all there. I'm positive. It is just difficult to get them. If the feds want to look at your texts thinking you are a terrorist, you can bet they can get the archive. I don't think the phone companies will give them to you if you ask, but if a court order says hand 'em over, they will.
I believe part of the patriot act requires the companies to keep them. It is really just a matter of storing the files on some machines in a warehouse somewhere.
Not far from me someone bought an old mall. The whole thing. They are filling the empty stores with computers and renting virtual "space". It is a pretty big outfit, and business is good.
Also, they are not really deleted from the phone. They are still in the phone. Deleting them simply frees that space up to be written over.
If you really, really want those texts stop using that phone now. Turn it off, pull the sim card out and install the sim card in another phone. your number stays the same. And some 17 year old techie kid will be able to recover the delete messages.
good luck.