code: public function del($userId, $callId)
      {
          $key = $this->getCacheKey($userId, $callId);
          $this->putInCache($key, $userId, $callId, null, self::STATE_PENDING_DELETE);
          return true;
      }
docs: Marks the the dataset to be removed from the storage.
  
  @param string $userId
  @param string $callId
  @return boolean