I have an unlocked Google Nexus One. In the last few weeks I have been in 4 different countries and tried with two different international sim cards in each. In both cases I don't have the option to search for mobile networks, although the phone seems to register signal strength. When trying to search for mobile networks none are available. Do I have to do something special to be able to use this phone as a phone and not just a device to play angry birds on?