When you initially launch the application you should see two options on first screen:
1. I'm new to Voxofon
2. I'm an existing customer
In case you already have an account you'll need to choose "Im an existing customer" option. You will be navigated to "This Mobile Number" screen when you need to enter the phone number which you've registered when creating your account - make sure to choose your country from the countries list before entering phone number. Then press "Save" button. You should be navigated to Sign in page, where all you need to do is enter your password and hit Login. That's it you're logged in!
In case you want to create a new account you should choose first option: "I'm new to Voxofon". On "This Mobile Number" screen enter your phone number (or any number you'd like to register), press "Save" button. On "Sign up" page enter email address and choose password. Press Sign up and you are registered!