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Maybe a stupid question but did you buy retail or from a 3rd party as some from 3rd party's can often be blocked by the networks because of unpaid bills by the seller, Example I buy a contract but stop paying then the phone included in this contract can be blocked
Either return your device to the place of purchase or contact Xperia care and have your device inspected http://www.sonymobile.com/global-en/support/contact-us/