Each country has both a country calling code used to dial into a country and an international access code or international call prefix used to dial out of the country.
The International Telecommunications Union approved 00 as the general standard some time ago - that is, all countries should work towards unifying on this code, and this was also adopted by the European Union. However, due to technological constraints, this is not currently feasible in all countries. Many member states also still use alternative dialing arrangements for calls to neighboring countries