title: How to check if phone number is already registered in firebase authentication using flutter
body: <p>So i am making a simple sign up and login screens in flutter application that uses phone authentication of firebase. For sign up im able to register new user, as the user provides his phone number and gets OTP. But for login i wanna check if the entered number is already registered. If so he gets otp and logs in or if not registered then asks to sign up first. </p>