I did a quick search through the forums but couldn't find what I was looking for. Granted, I really don't know what it's "officially" called, so I might have used the wrong search strings.
Does pfSense support source-port preservation? In other words, if my SIP phone sends a REGISTER message out UDP port 5060, my Linksys router assigns some high arbitrary port number (say, 50345) to the outgoing packet when it traverses it. Does pfSense do this? If so, can I disable this?
The option you are talking about is called "static port" in pfSense. You'll find it at firewall>nat, advanced outbound NAT tab. See http://forum.pfsense.org/index.php/topic,104.msg5876.html#msg5876 for the problem you have and how to set it up correctly.