SipXbridge is ITSP bridge geared towards the needs of the SIPX PBX system.
It aims to provide media and signaling gatewaying functionality  between
SIP endpoints inside a firewall /NAT and an external ITSP gateway.

The following wiki page will give you details on how to configure and run sipxbridge.

http://wiki.sipfoundry.org/display/oldxx/SipXbridge+Overview+and+Configuration