At last we can unlock Sony Ericsson A2 based phones (e.g. C902, C905, C903, K850, etc), and it's really easy! Also, this is a permanent unlock, just like one you would get at a phone shop! To verify if you have an A2 based phone, Click Here
TutorialFirst, fire up Aerix Client and choose your phone from the dropdown in the main window.
Remove the battery from your phone, then replace it. Hold down the 2 and 5 buttons WHILE plugging in the USB cable. You should see that a new hardware device is detected. Next, Aerix should show your phone info. Click Unlock, enter the Username: