For many modern SIP configurations this package is unnecessary. Please only install and use siproxd if it is absolutely required.
Siproxd is a proxy/masquerading daemon for SIP. 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 phones and soft phones (like kphone, linphone) to work behind an IP masquerading firewall or router.
The most useful thing siproxd does is allow multiple phones to use a static source port of 5060 when registering to the same remote PBX. If the PBX is local, this package should not be installed or used. Most remote PBX systems are OK with the phones having random source ports, but that was not the case many years ago. Unless the remote PBX is absolutely strict about the 5060 source port requirement for each phone, this package is not needed.
Under System > Packages
Find the siproxd package
Click to install, and confirm installation
Wait for it finish
Under Services > siproxd
Inbound interface will generally be LAN
Outbound interface will generally by WAN
Fill in any other values (where appropriate)
RTP port range (lower) should be an even number
You can find a list of known issues with the siproxd package on the pfSense bug tracker.