First of all make sure you purchased a Plan that includes calls and or texts to international destinations.
If this is the case, make sure you are dialing in international format.
Dial the + sign followed by the country code followed by the phone number without the starting 0.
Example to a french cellphone: +336…
33 is the country code.
Using the + sign followed by the country code is the standard international dialing method that should works worldwide. You should always save your contacts this way.
While this is not a recommended method and from USA only you can also dial 011 instead of using the + sign.
Example to a french cellphone: 011336…