I have a similar problem. I live in South Africa and I bought an unlocked 8820 from America (Original carrier:AT&T), It worked fine on my local carrier (Cell-C) until I upgraded it to BB OS v4.5 - It now does not pick-up any of the South African Mobile Networks including Cell-C. When I downgraded the OS back to BB OS v4.2 it worked fine again... Please help.