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This is normally an issue with the SIM card being dirty or of ill fitting - Is your card a manufacturer fitting card or one that has been cut down to size, Also check that it is free from defects and dirt/grease etc
Try cleaning and if that doesn't work then your next option is to get a new one and if that fails then ask the place of purchase to inspect your device or contact Xperia care http://www.sonymobile.com/global-en/support/contact-us/