One more clarification: is that port<5060 always the same, even at different tests? Or was the same only per each test?
You can try to define `event_route[tm:local-request] { ... }` and force send socket there (there is a function or can be done with assignment to $fs). I don't think there is anything else that can be done in kamailio, it may be a matter of OS/kernel settings or behaviour.