I know this is an old thread, but I've been having thir problem as well. Occasionally I would hunt around the web to try to find if someone has solved the issue. I'v just now found the solution (at least one that worked for me) and figured I would update this thread in case others are still having the problem. For me the solution turned out to be quite simple:
Create a new text message, to: 122
The message content should be:
Activate:dt=6
Send the message.
*Note: To get the '=', on the Alpha keyboard touch the symbol key. On the first symbol keyboard touch the right-pointing arrow key '->' to get to the second symbol keyboard.
Hope this helps.
Does it work for prepaid?