SIP Registration Issues Caused By Source Port Change

I've been testing FreeSBC to act as an SBC for our softswitch, and I have encountered an issue that I can't seem to fix.  I'm fairly confident the problem is the softswitch responds on a different port than it listens on, and the gateway is not associating the response with the original request even though the Call-IDs are the same.  The 401 Unauthorized message never makes it back to the client. Below is an anonymized excerpt from TCPDump and the gateway log.  I'm not sure where to go from here.

TCPDump

Code:

13:47:45.049980 IP <client_IP>.58015 > <SBC_WAN_IP>.sip: SIP: REGISTER sip:<SIP_domain> SIP/2.0
13:47:45.070560 IP <SBC_LAN_IP>.sip > <SoftSwitch_IP>sip: SIP: REGISTER sip:<SIP_domain> SIP/2.0
13:47:45.071970 IP <SoftSwitch_IP>.53753 > <SBC_LAN_IP>.sip: SIP: SIP/2.0 401 Unauthorized


gateway.log

Code:

16th, 13:47:45.060-0400 TBLV1 REG : <SoftSwitch_NAP>: HandleMessage:    <SoftSwitch_NAP> user:<SIP_User>@<SIP_Domain>
16th, 13:47:45.060-0400 TBLV1 REG : <SoftSwitch_NAP>: <SIP_User>: FindContact: Looking for '1_<SIP_User>_192.168.2.117_58015_1' -> <SIP_User>
16th, 13:47:45.060-0400 TBLV1 REG : <SoftSwitch_NAP>: <SIP_User>@<SIP_Domain> -> REGISTER: Forward to registrar (Retransmit (not active))
16th, 13:47:45.060-0400 TBLV1 REG : <SoftSwitch_NAP>: <SIP_User>: Remove user request timer: MjdjMjFhOWE1MjAyNjQwOTllODQ5NjQyMjFiOTFiYzM:8
16th, 13:47:45.060-0400 TBLV1 REG : <SoftSwitch_NAP>: <SIP_User>: Insert user request timer: MjdjMjFhOWE1MjAyNjQwOTllODQ5NjQyMjFiOTFiYzM:8
16th, 13:47:45.060-0400 TBLV1 REG : <SoftSwitch_NAP>: <SIP_User>: FindContact: Looking for '1_<SIP_User>_192.168.2.117_58015_1' -> <SIP_User>
16th, 13:47:45.060-0400 TBLV1 MON: SendSipOutOfCall: Register REQ from SRC:<SBC_LAN_IP>:5060 DST:<SoftSwitch_IP>:5060 (875 bytes)
16th, 13:47:45.082-0400 TBLV1 GW  : OnSipOutOfCall:   Register RSP from SRC:<SoftSwitch_IP>:53753 DST:<SBC_LAN_IP>:5060 (573 bytes) NAP:<SoftSwitch_NAP>
16th, 13:47:45.082-0400 TBLV1 REG : <SoftSwitch_NAP>: HandleMessage:    <SoftSwitch_NAP> user:<SIP_User>@<SIP_Domain>
16th, 13:47:45.082-0400 TBERR REG : <SoftSwitch_NAP>: CTBCAFSipRegistrationDomainForwarding::HandleRegisterResponse: No Registrar Found for this src IP/port. (82000009) CTBCAFSipRegistrationDomainForwarding.cpp:408

Hi

Would you contact support@telcobridges.com, please?