Or so t-mobile says.
http://www.msnbc.msn.com/id/7046776/
Basically they are saying that usually when you check your message and your not on your cell phone you have to enter your password , but if your dialing from your own number then it doesnt need to check your password because it knows your you. So all you have to do is spoof the number your calling and your in.
They say cell phone companies were notified of the problem months ago and most cell phones are still vulnerable..