libphonenumber\PhoneMetadata::getPreferredExtnPrefix PHP Method

    public function getPreferredExtnPrefix()
    {
        return $this->preferredExtnPrefix;
    }

 /**
  * Appends the formatted extension of a phone number to formattedNumber, if the phone number had
  * an extension specified.
  *
  * @param PhoneNumber $number
  * @param PhoneMetadata|null $metadata
  * @param $numberFormat
  * @param $formattedNumber
  */
 private function maybeAppendFormattedExtension(PhoneNumber $number, $metadata, $numberFormat, &$formattedNumber)
 {
     if ($number->hasExtension() && mb_strlen($number->getExtension()) > 0) {
         if ($numberFormat == PhoneNumberFormat::RFC3966) {
             $formattedNumber .= self::RFC3966_EXTN_PREFIX . $number->getExtension();
         } else {
             if (!empty($metadata) && $metadata->hasPreferredExtnPrefix()) {
                 $formattedNumber .= $metadata->getPreferredExtnPrefix() . $number->getExtension();
             } else {
                 $formattedNumber .= self::DEFAULT_EXTN_PREFIX . $number->getExtension();
             }
         }
     }
 }