[Serusers] Forwarding calls to a ShoreTel System

I will apologize in advance if this is the wrong forum or if this is an embarrassing newbie question. I am currently using an Trixbox/Asterisk system to connect a Rolm PBX to a ShoreTel phone system via SIP. The asterisk box doesn't correctly support the REFER method, so I'm hoping to use SER to proxy my SIP connections to allow the Rolm users to function correctly with transfers and conferencing. I have installed and configured my SER system using the getting started documentation. I have successfully interconnected my SER system with my Asterisk and with a Cisco 2801 and can place and route calls as I would expect. When I try to place a call through SER to my ShoreTel system, I get the standard "SIP/2.0 404 Not Found". If I compare captures of my successful SIP connection from my Asterisk environment to a capture of the SER attempt, the only real difference I see is that the INVITE TO: field has the IP of my SER system instead of the ShoreTel system. The actual INVITE uri is identifying the ShoreTel device, but the TO field has the SER address. The Asterisk connection has the address of the ShoreTel device in both places. I have tried to look through the docs and searched online, but I did not find any resources on this question. I would appreciate any feedback that could point me in the right direction. Here is a ser -V from my SER system version: ser 0.9.7-pre1 (i386/linux) flags: STATS: Off, USE_IPV6, USE_TCP, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, F_MALLOC, FAST_LOCK-ADAPTIVE_WAIT ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144, MAX_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535 @(#) $Id: main.c,v 1.197.2.1 2005/07/25 16:56:24 andrei Exp $ main.c compiled on 14:05:21 Jul 16 2008 with gcc 4.1.2 I route my calls to the ShoreTel system through the basic PSTN route from the getting started sample: route[5] { # ----------------------------------------------------------------- # PSTN Handler # ----------------------------------------------------------------- rewritehost("my.IP.addr.here"); # INSERT YOUR PSTN GATEWAY IP ADDRESS avp_write("i:45", "inv_timeout"); t_on_failure("1"); route(1); } Thank you for any feedback in advance.