This simply means that the mobile number you're trying to use is already registered to another existing account. You may already have an account that you've overlooked, accidentally created or deactivated, using that particular mobile number.
This, therefore, means that you can log in to the existing account where your mobile number is originally registered to. And if you to need reset its password, you can easily do so through this page.
You may also contact our Support Team to help you withdraw the number in your deleted account.
Please take note that this notification also applies to e-mail addresses.