Is it possible to use mobile minutes on a tablet
It depends on the tablet. Not all 3G tablets have the facility to make voice calls (just like a phone), but some do. Some have the hardware and OS support, and you just need to install a dialer app to use it; some don't even have the right hardware. The Nexus 7 in particular doesn't.

You should check with the seller or the manufacturer's website before you choose a device.