To check in Xperia X10 which simlock is in, please do:
1. Start up the phone and press the Menu key ( ) and Back key ( ) as follows:
2. Pressing Key Sequences: Menu Back Back Menu Back Menu Menu Back
Note: Please make sure the phone have SIMLOCK when pressing these keys to get into the Service menu.
Also You can check there is counter of wrong code is blocked.
For new Sony and Xperia Phones
Switch on your phone without SIM card Compose *#*#7378423#*#*
Select Service Info and then Simlock If X is with Network and it is not 0 you can unlock your phone via code
If X is 0 your counter is locked and unlocking via code is not possible.
Note: in case of Not Unlock issue please make a video while showing Remaining counter (Follow how to check remaining Counter" Compose without Sim card *#*#7378423#*#* Select Service Info and then Simlock") imei and error on Screen Upload at youtube.com Click on Publish Send imei and video link