kamailio.cfg: reset $du for voicemail re-routing in failure route

(cherry picked from commit c4b29f6fac6155b9b087737df5956b687ff4e36a)

etc/kamailio.cfg

@@ -921,6 +921,7 @@ failure_route[MANAGE_FAILURE] {
 	# serial forking
 	# - route to voicemail on busy or no answer (timeout)
 	if (t_check_status("486|408")) {
+		$du = $null;
 		route(TOVOICEMAIL);
 		exit;
 	}