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Do you have the option with power button ends call?
If the power button doesn't work at all I would suggest that you 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 phone/tablet software > Press the blue link that says "Repair phone/tablet".
If the issue still persists, I suggest that you have your 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/