/**
  * you can define default filters here
  * @param Tinebase_Model_Filter_FilterGroup $_filter
  */
 protected function _addDefaultFilter(Tinebase_Model_Filter_FilterGroup $_filter = NULL)
 {
     $lines = Sipgate_Controller_Line::getInstance()->search(new Sipgate_Model_LineFilter());
     if ($lines->count()) {
         $aclFilter = new Tinebase_Model_Filter_Text(array('field' => 'line_id', 'operator' => 'in', 'value' => $lines->id));
         $aclFilter->setIsImplicit(true);
         $_filter->addFilter($aclFilter);
     } else {
         $_filter->addFilter(new Tinebase_Model_Filter_Text(array('field' => 'entry_id', 'operator' => 'equals', 'value' => 'impossible')));
     }
 }