public function customers_list()
 {
     $screen = isset($_GET['screen']) ? $_GET['screen'] : '';
     switch ($screen) {
         case 'customer_notes':
             WC_CRM_Screen_Customers_Edit::display_notes($_GET['c_id']);
             break;
         case 'order_list':
             WC_CRM_Screen_Customers_Edit::display_order_list($_GET['c_id']);
             break;
         case 'product_list':
             WC_CRM_Screen_Customers_Edit::display_products_list($_GET['c_id']);
             break;
         case 'email':
             WC_CRM_Screen_Activity::display_email_form();
             break;
         case 'phone_call':
             WC_CRM_Screen_Activity::display_phone_call_form();
             break;
         default:
             if (isset($_GET['c_id']) && !empty($_GET['c_id'])) {
                 WC_CRM_Screen_Customers_Edit::output($_GET['c_id']);
             } else {
                 WC_CRM_Screen_Customers_List::output();
             }
             break;
     }
 }