From what you describe, the reply should be going back to the sender. Are you absolutely sure that it's not? If so, are there any other actions you could be taking somewhere to drop it, such as in an onreply_route?

ACKs to negative final replies are hop-by-hop, so the ACK you're seeing directly from the proxy to the UAS is normal. 

--
Alex Balashov | Principal | Evariste Systems LLC
303 Perimeter Center North, Suite 300
Atlanta, GA 30346
United States

Tel: +1-800-250-5920 (toll-free) / +1-678-954-0671 (direct)
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/

Sent from my BlackBerry.
From: Alexandru Covalschi
Sent: Tuesday, December 15, 2015 05:03
To: Kamailio (SER) - Users Mailing List
Reply To: Kamailio (SER) - Users Mailing List
Subject: [SR-Users] Relaying failure codes back to initial server

Hello everyone!
I need to relay 486/408/... other failure codes back to initial INVITE server. Here http://lists.sip-router.org/pipermail/sr-users/2010-November/066382.html is recommended just to exit failure_route, but that didn't work for me. I need that to let Freeswitch know which cause has ended the call. Now Kamailio just sends ACK to endpoint on receiving 486 BUSY. Would you kindly tell me how to achieve that?
Thanks in advance
--
Alexandru Covalschi
ABRISS-Solutions
VoIP engineer and system administrator
phone: +37367398493
web: http://abs-telecom.com/