The services provided by professionals in particular field. For example, a company may charge a certain sum of money for the certain tasks their consultant accomplished for the company who ordered the service.
The basic idea behind a Call Shop business is to have a place where customers can make local, long distance or international phone calls. The main sources of revenues will come from the minutes customers are going to spend on the phone. So it all depends on the rates that you are going to setup, just make sure that they are lower than the ones offered by the providers of international call services.