The errors is not due to re-transmission, but packet ordering.
It seems that, in high loaded systems, kamailio proxy takes too
much time to process some packages causing an unexpected order.
Increasing the wait time from 500ms to 600ms give 100ms more to the
proxy to process the packet and send it out in the expected order.
Change-Id: Idccd6c7b95c428cbee519bb6cf779c6a8dfc3e82