My phone not registering to 2G (GSM only) network
As it seems you have tried resetting to factory defaults and re-flashing same update, I believe the causes might be caused by updated baseband. I would try reflashing different original firmware. You can also flash baseband only via flashtool so maybe it might be worth trying to flash baseband from the firmware that worked. YMMV.

You can find links to download FTF (original firmware) for your device:


Build 4.1.B.0.587 - http://d-h.st/Vft
Build 4.1.B.0.626 - http://goo.gl/hAfwI
Build 4.1.B.0.631 - http://d-h.st/iVF


Sources &amp; more info:


http://forum.xda-developers.com/showthread.php?t=2047914
http://forum.xda-developers.com/showthread.php?t=1952333
http://www.flashtool.net/index.php