"Llamar gratis por Internet" means free Internet call. There are many ways to make a phone call for free via the Internet, including PC to phone, app to phone, PC to PC and app to app.
Services like Google Voice, Viber, Tango, Skype and others utilize the Internet to make free calls to and from telephones and between computers and mobile devices. Utilizing the free data transfer ability provided by the Internet along with technology that translates that data into phone signals, users can make calls to and from anywhere in the world for free. Signing up for most of these services is also free.