What so you mean by international codes? What country are you from? Are you using your exit dialing digits or +? Always use +countrycode then phone number, no matter where you are.
For instance, in the USA, we use 011 then country code to dial out, but if I were in Europe and dialed 011 then country code, it would not work. And they use 00 then country code in a Europe and much of the world. But dialing that while in the USA would not work.
Always use +countrycode. So to call the USA from anywhere, it's +1. To call Spain from anywhere, it's +34.