How to fix auth/missing-phone-number in Firebase

Add a simple check to ensure the phone number is not empty or null before calling the Firebase authentication method:

const phoneNumber = userInput.trim();

if (!phoneNumber || phoneNumber.length === 0) {
  console.error('Phone number is required');
  setError('Please enter a valid phone number');
  return;
}

try {
  const confirmationResult = await signInWithPhoneNumber(
    auth,
    phoneNumber
  );
} catch (error) {
  console.error('Authentication failed:', error.code);
}

This prevents the auth/missing-phone-number error by validating input before the Firebase call.

Firebase requires phone numbers to be formatted with a country code prefix (e.g., +1 for US, +44 for UK). Make sure your input validation includes this:

const phoneNumber = userInput.trim();

// Ensure phone number starts with +
if (!phoneNumber.startsWith('+')) {
  setError('Phone number must include country code (e.g., +1 for US)');
  return;
}

if (phoneNumber.length < 10) {
  setError('Phone number is too short');
  return;
}

await signInWithPhoneNumber(auth, phoneNumber);

Before implementing phone authentication, you must enable it in your Firebase project:

1. Go to the Firebase Console (console.firebase.google.com)
2. Select your project
3. Navigate to Authentication section
4. Click on "Sign-in method" tab
5. Find "Phone Number" in the list
6. Click the toggle to enable it
7. Click "Save"

Without this enabled, all phone authentication attempts will fail. This is a project-level configuration, not code-level.

Wrap your phone authentication call in try-catch and handle specific error codes:

try {
  const phoneNumber = form.phoneInput.value.trim();
  
  if (!phoneNumber) {
    throw new Error('Phone number is required');
  }
  
  const confirmationResult = await signInWithPhoneNumber(
    auth,
    phoneNumber
  );
  
  // Store for later use
  window.confirmationResult = confirmationResult;
  
} catch (error: any) {
  switch(error.code) {
    case 'auth/missing-phone-number':
      setError('Please enter your phone number');
      break;
    case 'auth/invalid-phone-number':
      setError('Phone number format is invalid');
      break;
    case 'auth/quota-exceeded':
      setError('Too many requests. Please try again later');
      break;
    default:
      setError(error.message);
  }
}

Firebase allows you to register test phone numbers in the console to test without sending real SMS messages:

1. In Firebase Console, go to Authentication > Sign-in method
2. Expand the "Phone" provider
3. Click on "Phone numbers for testing"
4. Add a test phone number (e.g., +1 650-555-3434) and verification code
5. In your app, when you use a registered test number, entering the test code will authenticate without sending SMS

Example test numbers you can use:
- +1 650-555-3434 (US format)
- +44 7444 555666 (UK format)
- +81 90-1234-5678 (Japan format)

Test numbers prevent quota issues and let you verify your authentication flow works correctly.