First of all I suggest that you try with another SIM card in your phone.
If you experience the same problem with another SIM card you could try to repair the phone software using PC Companion:
http://www.sonymobile.com/update
In PC Companion, press start on Support Zone > Start on Update the phones software > "Repair phone".
If the problem still persists, you may have the phone examined by a service center. Please contact your local Xperia support team for more information:
http://www.sonymobile.com/global-en/support/contact-us/
