When you are entering your phone number (Whether it's when you are signing up or on your account), you need to make sure that you are using the international format without any spaces or other characters, just the numbers.
EG 15551111. You do not need to enter the + or the 00.
Once you updated or verified your number, you can go to the "Settings" menu and enable Two-factor verification (2FA). It provides further security to your account.
Want to enable Two-factor verification (2FA) via an Authenticator App: Tap here.
Want to enable SMS Two-factor verification (2FA): Tap here.