[Kamailio-Users] correct usage of t_reply

I'm am attempting to return a 503 "Service Unavailable" after my kamailio server sends the provisional response to customer UA if we cannot find a route for the call. I am using: t_reply("503", "Service Unavailable"); However this results in: ERROR:tm:w_t_reply: failed to send a t_reply to a message for which no transaction-state has been established Reading up on t_reply, it clearly states: "Sends a stateful reply after a transaction has been established." So, I guess thats means even though we have received the invite, and send 100 Trying back, a transaction does not actually exist. So, my question is, how do I tell the sending UAC that we are 5XX so that it can stop sitting there waiting for session progress? Thanks for any help. David