I have looked around for an answer for this question, but I seem to be turning up nothing. Here is my dilemma. I work in an office in a basement with No ATT Cell Service. I get all my left VM messages sent to Google Voice Mail, so I get them thru the GV web interfact, but I haven't figured out how to do this sort of thing with my SMS Messages. I know there is probably an App to do the forwarding, but that would require the messages getting to my Iphone in the first place, which is why I was hoping there could be a forwarding feature on ATT's side.
There is a website http://store.handango.com which has various software for use with mobile media. One is called "MessageForward" which automatically forwards a copy of all incoming SMS text messages to any email address you provide. Check it out to see if there is a program there that will support your phone. Hope this is what you're looking for.