I wanted to ask those who know if Kamailio's behavior I'm facing is expected or I should make some improvements to the configuration. Kamailio version is 5.1.0.

I have a route where RTPEngine parameters are being collected and  rtpengine_offer() is called. After that t_on_reply("REPLY_SIP_TO_SIP"); followed by the t_on_failure("FAILURE_SIP_TO_SIP"); are used. The idea is to process all responces except 415 or 488 from UAC as usual in onreply_route[REPLY_SIP_TO_SIP] and use failure_route[FAILURE_SIP_TO_SIP] to update SDP with rtpengine_offer() if necessary.

onreply_route[REPLY_SIP_TO_SIP] just goes to exit; if $rs equals  415 or 488. This works fine with Htek phone which sends 100, 180 and then 488. But I can not see  failure_route[FAILURE_SIP_TO_SIP] execution for calls to Zoiper which replies with 100 and immediately 415.

t_on_failure(failure_route) documentation says: "Sets failure routing block, to which control is passed after a transaction completed with a negative result but before sending a final reply." and to be honest I don't really understand how lacking of responce prevents  failure_route from executing.