Length of time SMS/MMS messages are saved on "server"?

This might come off as a random question, and might be more of an AT&T question, but these boards have always been helpful. I recently took a trip overseas, and left my iPhone on "Airplane Mode" while abroad, as to not incur any charges. Upon arriving back in the United States today, I switched my iPhone back to regular connection, allowing it to connect to 3G perfectly. Oddly, I have not received any text messages from the past two weeks I was away, and know that I should have at least a dozen coming in.

Is there a certain amount of time that text messages stay on AT&Ts server, and if you don't have your phone connected in that timeframe, you never get them? Any help would be appreciated!