Hello, Alex!
Fisrt of all, thanks for you help with this problem!
I don't think that this will help me. I can see that this parameter is enabled by default and in fact it is working in some cases. One case where this works is when the INVITE is sended for more than one endpoint (when the same user is registered in many devices). When someone answers the INVITE, Kamailio sends a CANCEL message to the other endpoints and it has the Reason header showing that the call was answered elsewhere.
I want to add the CANCEL Reason header when the timer expires, so kamilio will not receive a final replay to that transaction.