getPhoneNumber() {
    var countryData =
        firebaseui.auth.data.country.getCountryByKey(this.countryId);
    if (!countryData) {
      throw new Error('Country ID ' + this.countryId + ' not found.');
    }
    return '+' + countryData.e164_cc + this.nationalNumber;
  }
@return {string} The full phone number.