Upgrade DB2020 CID 49 and 51 to CID 52
Quite a few SonyEricsson phones need their CID (security version) upgraded to CID 52 before unlocking. The software to use with this is called XS++.
As soon as you connect, it'll ask you if you want to upgrade to CID 52. Click yes. You can now unlock the handset by following the other unlocking guides. Click the picture to download the software.