I had my 3GS iphone unlocked when I traveled to Asia for work so I could use a local sim card and save on roaming charges. I turned off the data roaming plan, inserted Thai, Cambodian and Vietnamese prepaid sim cards in and proceeded to run the credit to $0.00 on all three, so I know they were working. So I was pretty horrified to find over $900 in roaming charges when I looked at my at&t bill. This was all from calls not from data transfer as I've seen mentioned before. What did I do wrong?