What tablet? Does this tablet have mobile data service? What carrier are you with?
A tablet typically will not have the ability to make a phone call unless you have the ability to do some kind of VOIP like using Google Voice or a Project Fi number with Hangouts.
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You can't make calls because it's not a phone. You will need to install a phone application that works over the internet, as opposed to a cell phone using wireless data. Good luck.