I found another bug in Callback module. Leaving the "Delay Before Callback" field blank or putting non-numeric letters will cause SQL script error. The workaround is to add the following line.
$sleep = (is_numeric($sleep) && $sleep >= 0) ? ceil($sleep) : 0;
under
if(empty($description)) $description = ${$goto0};
in both function callback_add and callback_edit of /var/www/html/admin/modules/callback/functions.inc.php.
Hope everyone can use this module without that problem anymore.