[OpenSIPStack] B2BUpperReg and back door contact issue

[Jack D. <455...@gm...>]

I have a problem where OSBC is running as B2BUpperReg and successfully
registers a Linksys
SIP phone (using OSBC as the outbound proxy) to the upstream registrar (a
B2BUA gateway).

However, in the mangled REGISTER message OSBC sends, it sets the contact
header to
its IP address but it's not using port 5060, but the back door port (65080).

Now what happens is that INVITES from this SIP phone routes through OSBC in
B2BUA mode
but OSBC uses source port 5060 to send that invite to the gateway instead of
using the backdoor
port as the gatweay has in it active registration tables.  The call is
therefore denied with a 403 Forbidden.

Is this a bug in OSBC ?  -or- is there an option I am missing to instruct
OSBC to use the backdoor
port in requests to the gateway ?
Better, I would rather like OSBC to send the REGISTER with the default  port
5060 in the contact.

Is there documetation available that explains in details the purpose and
role of the back door thread ?

Calls from the gateway -> OSBC -> SIP phone are working just fine.

Thank you,
JD