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I think you have the model with wi-fi only.See here more:
http://www.sony.co.uk/electronics/xperia-tablet-z/sgp311-sgp312-sgp321/specifications
@alexdon is right. SGP312 is the Wi-Fi only version of Xperia Tablet Z so it doesn't support any SIM card. If you want support for SIM card you will have to get the SGP321 version of the tablet.