Does this happens in every call? If that's the case it might be something with the microphone that's faulty. You could try to repair the phone software via PC Companion to see if it's software related. If the problem still persists you should send in the phone to service for repair. Contact your local Xperia Support for more information:
http://www.sonymobile.com/update
http://www.sonymobile.com/global-en/support/contact-us/
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