The International number is a private and anonymous number that you can use to call and text anyone around the world. A different number will appear for every different person you call or text, allowing you to stay completely private.
To find out your number: You need to text someone FIRST and then your contact will be able to see your number to text you back. Until you text or call someone, the International Number is not known.
Once you have texted or called someone, a number is randomly assigned and becomes the permanent number between you and the recipient.
For a more permanent number that you can keep or share: You can select a Premium number for this. Tap on top left button in (Menu) -> Get Premium number and choose a premium number.