How to check if a user phone number exists using firebase auth? [duplicate]

Question

I'm building an app where i prompt the user to authenticate with their phone number in the first place. if the user phone number doesn't already exist he's redirected to a page with username and password. IF he exists, he's prompted to the password page directly.

How i can check if the user phone number already exists (or not) before knowing which page he will be sent?

i did check the docs, i found this solution

getAuth()
  .getUserByPhoneNumber(phoneNumber)
  .then((userRecord) => {
    // See the UserRecord reference doc for the contents of userRecord.
    console.log(`Successfully fetched user data:  ${userRecord.toJSON()}`);
  })
  .catch((error) => {
    console.log('Error fetching user data:', error);
  });

But i need to it run in a backend server. What i want is something similar but that i will use in the front ( i'm using react ). Firebase offers plenty of methods in the front-end for authentication of the user. Where can i find one that fetches it ?