Phone number sign-in 

We have already seen phone number sign-in earlier. This method allows developers to customize the user interface and functionality. The following code makes use of most of the callbacks:

private void setPhoneVerificaton() {
        verificationCallbacks =
                new PhoneAuthProvider.OnVerificationStateChangedCallbacks() {
                    @Override
                    public void onVerificationCompleted(
                            PhoneAuthCredential credential) {
                        signInWithPhoneAuthCredential(credential);
                    }
                    @Override
                    public void onVerificationFailed(FirebaseException e) {
                        if (e instanceof FirebaseAuthInvalidCredentialsException) {
                            // Invalid request
                            Log.d(TAG, "Invalid credential: "
                                    + e.getLocalizedMessage());
                        } else if (e instanceof FirebaseTooManyRequestsException) {
                            // SMS quota exceeded
                            Log.d(TAG, "SMS Quota exceeded.");
                        }
                    }
                    @Override
                    public void onCodeSent(String verificationId,
                                           PhoneAuthProvider.ForceResendingToken token) {
                        phoneVerificationId = verificationId;
                        resendToken = token;
                       
                    }
                };
    }

The important callback methods of phone number sign-in are: 

• onVerificationCompleted(): This can be triggered when the verification happens automatically without manual input
• onVerificationFailed(): This callback represents a wrongly entered OTP and other errors
• onCodeSent(): After code is sent to the number, using this method we can update the UI and other functionalities