Hide Your Account. By default, your phone number is hidden to everyone but your contacts.

The only way for a user to see your profile is to know your phone number, username, or if they are in a group chat with you.

In your Privacy Settings, you can make your phone number visible to Nobody, which opens an extra setting: Who Can Find Me By My Number?

By setting Who Can Find Me to My Contacts, if anyone other than your contacts searches for your phone number, it will appear as if your account does not exist.