code: public function GetValidVouchers( $customer_id, $event_id, $contact_person_id = null ) {
  		$params = array();
  		if ( isset( $contact_person_id ) ) {
  			$params['contactPersonId'] = $contact_person_id;
  		}
  
  		return parent::GET(
  			"/$customer_id/ValidVouchers/$event_id",
  			$params,
  			get_called_class() . '|' . __FUNCTION__
  		);
  	}
docs: @param integer $customer_id
  @param integer $event_id
  @param integer|null $contact_person_id
  
  @return mixed