Tablets being able to make calls
The Galaxy Tab 10.1 can make 2G/3G calls and calls over Wifi with Voip apps like Skype or Tango.

The Nexus 7 can only make calls over Wifi with Voip apps as I already mentioned. There would be no easy way of getting the Nexus 7 to make 2G/3G calls as it would require adding a radio to the tablet.