No, I do not have a trunk setup explicitly for the DSIProuter. (I'm using the FreePBX pass through case) If I understand it correctly, the DSIProuter is simply acting as a pass through proxy. This means registrations too. I think I'm using a slightly different use case than you.
My current issue is a weird one. I'm able to place calls from a DSIP connected client to an phone directly connected to the PBX. RTP seems to work, no timeout afte 30/60 seconds. However, if I call from the directly connected phone to the DSIP connected phone, I get no audio and the connection drops after 30 seconds with error messages similar to the following:
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: INFO: [/etc/kamailio/kamailio.cfg:1620:RELAY] [49254b96-7840-4300-b4d0-f1d479e06a9b:BYE:<null>] Attempting to route call to sip:99...@10.0.0.140:44865;transport=TLS;ob
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: ERROR: [<null>:0:<null>] [49254b96-7840-4300-b4d0-f1d479e06a9b:BYE:<null>] <core> [core/tcp_main.c:2951]: tcpconn_1st_send(): connect
172.16.8.23:56494 failed (RST) Connection refused
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: ERROR: [<null>:0:<null>] [49254b96-7840-4300-b4d0-f1d479e06a9b:BYE:<null>] <core> [core/tcp_main.c:2960]: tcpconn_1st_send():
172.16.8.23:56494: connect & send for 0x7fede7e5cda0 (sock 21) failed: Connection refused (111)
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: ERROR: [<null>:0:<null>] [49254b96-7840-4300-b4d0-f1d479e06a9b:BYE:<null>] tm [../../core/forward.h:292]: msg_send_buffer(): tcp_send failed
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: WARNING: [<null>:0:<null>] [49254b96-7840-4300-b4d0-f1d479e06a9b:BYE:<null>] tm [t_fwd.c:1588]: t_send_branch(): sending request on branch 0 failed
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: INFO: [/etc/kamailio/kamailio.cfg:1620:RELAY] [49254b96-7840-4300-b4d0-f1d479e06a9b:ACK:<null>] Attempting to route call to sip:99...@10.0.0.140:44865;transport=TLS;ob
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: ERROR: [<null>:0:<null>] [49254b96-7840-4300-b4d0-f1d479e06a9b:ACK:<null>] <core> [core/tcp_main.c:2951]: tcpconn_1st_send(): connect
172.16.8.23:56494 failed (RST) Connection refused
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: ERROR: [<null>:0:<null>] [49254b96-7840-4300-b4d0-f1d479e06a9b:ACK:<null>] <core> [core/tcp_main.c:2960]: tcpconn_1st_send():
172.16.8.23:56494: connect & send for 0x7fede7e5cda0 (sock 21) failed: Connection refused (111)
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: ERROR: [<null>:0:<null>] [49254b96-7840-4300-b4d0-f1d479e06a9b:ACK:<null>] <core> [core/forward.h:292]: msg_send_buffer(): tcp_send failed
Aug 4 10:02:20 dsip001 /usr/sbin/kamailio[50581]: ERROR: [/etc/kamailio/kamailio.cfg:1622:RELAY] [49254b96-7840-4300-b4d0-f1d479e06a9b:ACK:<null>] sl [sl_funcs.c:414]: sl_reply_error(): stateless error reply used: Unfortunately error on sending to next hop occurred (477/SL)
I'm not sure why Kamailio is trying to connect back to itself on that port (172.16.8.23 is the DSIProuter). It seems like instead it should be attempting to connect to the IP address in the location table; the public/port pair that was recorded on successful registration.