I just bought an unlocked Blackberry 7780. I currently have service with AT&T. My SIM card works in other phones. When I put the SIM card in this phone, I can access my address book and when I go to Options, Network, it shows AT&T. I can't make calls though. Up in the corner where the signal strength is, there are no bars, and it says SOS. How do I get out of this mode?