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 work via a masquerading firewall (NAT).
It allows SIP software clients (like kphone, linphone) or
SIP hardware clients (Voice over IP phones which are
SIP-compatible, such as those from Cisco, Grandstream or
Snom) to work behind an IP masquerading firewall or NAT
router.

SIP (Session Initiation Protocol, RFC3261) is the protocol
of choice for most VoIP (Voice over IP) phones to initiate
communication.
By itself, SIP does not work via masquerading firewalls as
the transferred data contains IP addresses and port numbers.
There do exist other solutions to traverse NAT existing
(like STUN, or SIP aware NAT routers), but such a solution
has its disadvantages or may not be applicable to a given
situation.
Siproxd does not aim to be a replacement for these solutions,
however in some situations siproxd may bring advantages.