KAZOO-2363: do not remap the valid 6xx codes

kamailio/default.cfg

@@ -556,7 +556,7 @@ failure_route[INTERNAL_FAULT]
     remove_hf_re("X-.*");
 
     # change 6xx to 4xx
-    if (t_check_status("6[0-9][0-9]")) {
+    if (t_check_status("6[0-9][0-9]") && !t_check_status("600|603|604|606")) {
         $var(new_code) = "4" + $(T_reply_code{s.substr,1,0});
         xlog("L_INFO", "$ci|log|sending 6XX reply as $var(new_code) $var(reply_reason)");
 	    t_reply("$(var(new_code){s.int})", "$var(reply_reason)");