Siproxd is a proxy/masquerading daemon for the SIP protocol.
It handles registrations of SIP clients on a private IP network
and performs rewriting of the SIP message bodies to make SIP
connections possible via a masquerading firewall.
It allows SIP clients (like kphone, linphone) to work behind
an IP masquerading firewall or router.

WWW: http://siproxd.sourceforge.net/