SOS on my unblocked blackberry! HELP!?

Ok, so I have an unlocked Blackberry 8310. It's originally for at&t, but I got it unlocked so i can use it for any network. (I have net10). I put in my SIM card and it reads its fine. It shows my number on the phone, so I know my SIM card is in correctly. The problem is, in the top right corner where the signal bars usually are, it reads "SOS". I have no idea why. I wiped the handheld, I've tried to take the battery out and put it back in, as well as the SIM card. I also tried turning it on and off several times. What do I have to do for it to work?!! HELPP!?