getPhoneAuthDefaultNationalNumber() {
    const signInOptions = this.getSignInOptionsForProvider_(
        firebase.auth.PhoneAuthProvider.PROVIDER_ID);
    // Check if loginHint passed. If so, get the national number from there.
    // If no defaultNationalNumber passed, use this value instead.
    let defaultPhoneNumber = null;
    if (signInOptions && typeof (signInOptions['loginHint']) === 'string') {
      defaultPhoneNumber = PhoneNumber.fromString(
          /** @type {string} */ (signInOptions['loginHint']));
    }
    return (signInOptions && signInOptions['defaultNationalNumber']) ||
        (defaultPhoneNumber && defaultPhoneNumber.nationalNumber) || null;
  }
Returns the default country to select for phone authentication.
@return {?string} The default naional number, or null if phone auth is not
    enabled.