The blacklist represents itself a list or register of entities which are denied privileges, service, mobility, access and admission due to these or other reasons. This function blocks all connections from the subscribers who are in the list.
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.