Fiat, netTALK is outgoing-only, it cannot answer incoming calls or be assigned a phone number. You get 50 minutes of free calling per month, per the FAQ in the app. There also used to be a limit of 30 minutes per call, but that may no longer apply.
Obviously, netTALK isn't gonna replace your phone. It's a handy tool for certain situations, such as calling ahead for reservations on a vacation to the other side of the US/Canada border. My guess: they want to sell you a VOIP box for your home or business, and the free app is a good way to demo their quality of service and build brand recognition.
I used it to test a new home phone system, by calling my own number to assess the voice quality and exercise all the features.
milliHelen: amount of beauty required to launch one ship.