If you are getting a message that your phone number is already registered, chances are you have an account already and just need to log in.
Try this to check:
- Tap on the person icon to open the left menu
- At the top of the page you should see the option to "Register or Sign In"
- Enter your email and password, or select Google+ or Facebook, if you originally used one of those.
If you continue to have trouble, contact our support team for assistance: firstname.lastname@example.org