title: Firebase login with phone & password
body: <p>I want the user to be able to login with phone and password.</p>
  
  <p>If the user enters the app first time, he es registered by validating his phonenumber, thats the current state at firebase. But I dont want to always send a sms code to the user every time he wants to login.</p>
  
  <p>Now in the next step of the authentification the user should enter an email and a password, so I can link the credentials with the phone. The user is now able to login with email and password and with phone and verification code. </p>
  
  <p>But I want to have the function that the user can login with phoneNumber and password. So I can write a cloud function for "doesPhoneNumber" exist which goes through the user-db and tells me if the user can login or have to sign up. </p>
  
  <p>The next function is now my actual problem, i want to send the phone number and the password to a extra cloud function to receive the authToken but how can I verify if the credentials are correct within that function? </p>