It uses minutes, it doesn’t go over the data plan. It calls a number that GV converts to your number. (The number GV dials makes money for Google, you’ll notice that it’s in “weird” area codes that costs the carriers more money to terminate to.)
What I did with my plan was use Google Voice, IPKAll and Callwithus to have a 100% totally free system. IPKall gives you a free telephone number, Callwithus gives you a SIP plan, you add your IPKall nummber to GV, have your phone connect to Callwithus and use GV+ to make your outgoing calls for free calls to the US, from anywhere in the world.