The international call prefix depends on the country you are calling from, e.g. 00 for most European countries, and 011 from North America. For domestic calls (within the country), 0 must be dialed before the area code. The prefix for international calls from Croatia is 00 (e.g. for an United States number 00 1 ... should be dialed).
An example for calling telephones in Bjelovar (area code 043) is as follows: