National Calling refers to calls made to phone numbers in the same country as the your own phone number. The app automatically sends these calls to the native iPhone dialer by default. If you are charged long distance within the same country by your regular carrier, or want to call over WiFi when you're abroad, you can turn on the option to use Rebtel to route your call instead. The National Calling option is turned off by default, so you have to go to the iPhone's Settings app to change it.