code: func FormatNationalNumberWithPreferredCarrierCode(
  	number *PhoneNumber,
  	fallbackCarrierCode string) string {
  
  	pref := number.GetPreferredDomesticCarrierCode()
  	if number.GetPreferredDomesticCarrierCode() == "" {
  		pref = fallbackCarrierCode
  	}
  	return FormatNationalNumberWithCarrierCode(number, pref)
  }
docs: // Formats a phone number in national format for dialing using the carrier
  // as specified in the preferredDomesticCarrierCode field of the PhoneNumber
  // object passed in. If that is missing, use the fallbackCarrierCode passed
  // in instead. If there is no preferredDomesticCarrierCode, and the
  // fallbackCarrierCode contains an empty string, return the number in
  // national format without any carrier code.
  //
  // Use formatNationalNumberWithCarrierCode instead if the carrier code
  // passed in should take precedence over the number's
  // preferredDomesticCarrierCode when formatting.