Try clearing the data for Phone under Settings > Apps > All > find Phone & tap it > Tap clear data. See if that resolves the issue. If it doesn't then I advise booting the phone in to Safe Mode to see if the same issue occurs then.
To boot your phone in safe mode press the power key until the Power Off message appears > hold your finger on Power Off for 2-3 seconds until 'Reboot to safe mode?' appears > tap OK. To exit Safe Mode just power the phone off & then on normally.
Let us know how it goes.
- Official Sony Xperia Support Staff
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