How to implemet whatsapp type phone number authorization in Firebase

Question

Following is what I am trying to achieve:

• User installs the app
• On its first launch, user will be asked to specify phone number and Name
• Phone number gets verified by invoking REST API of OTP service providers
• Once 2 Factor Authentication gets complete, I will create the user in Firebase database with its mobile number and Name as attribute to unique phone number

Problem: Now, what I am not getting is how to authenticate user with only phone number, as I want to track all his activities under his/her mobile number.

function showLoginBox() {
  ...
  // Do whatever DOM operations you need to show the login/registration box.
  $("#registerButton").on("click", function() {
    var email = $("#email").val();
    var phone = $("#phone").val();

   // Invoke REST API to generate OTP/ Miss call
   // Read OTP or user enters the OTP
   // Invoke REST API to complete 2FA (Two factor authentication)
   // Add the user information to Firebase
    authClient.createUser(email, phone, name, function(error,  user) {
      if (!error) {
        doLogin(user);
      } else {
        alert(error);
      }
    });
  });
}

Now doLogin() function will be passed user object and I can get the user details, but how do I login now.. as I cannot invoke email/password combination or Anonymous login. I even saw the JWT (JSON web token) thing, but it is mentioned under custom login, not sure whether it make sense

Can I do following: - Save the verified mobile number (after 2FA) into local database (window.localstorage) - Read local storage and send the mobile number with every subsequent call

But unless, I will invoke the authentication, how do I implement security/authorization, so as this mobile number can access only its related data.

Can anyone help me to figure out following things: - How do I login the user with his mobile number only - How can I track each and every activity performed by the user by his mobile number (Here mobile number is his user id)

Thanks in advance.