I'm sorry you're having problems sending international text messages. I'd recommend reviewing our What do I need to do before using my phone to call internationally? document. It will ensure your account is setup correctly and that you are typing in the number correctly.
Let me know if this helps!
I will need a little more information in order to better assist you. Have you sent an international message from this phone before? When you attempt to send the messages, what error message do you see?