In this tutorial, we learn how to use MMS on T-mobile or ATT iPhone using 3.1 or lower. First, you need to make sure you have iTunes down and it's not running on your computer at all. After this, go to the start menu, then click on the run. Next, type in "command" and hit enter. Then, type or copy and paste "C:/Program Files/iTunes/iTunes.exe/setPreflnt carrier-testin 1" into the command console that comes up. Then, hit enter and reboot your iPhone. When this reboots, go to the settings menu, then messages, and mms messaging. After this you'll be finished!