asterisk users - Oct 2005 - IAXy Port number. Repost

The file used with a Digium IAXy device: "iax.conf"
has the line: "port=5036" (I also use the
"bindaddr=192.168.1.91" entry)
but when Asterisk talks to the IAXy device it
used port 4569 (from "tcpdump"). How are the port numbers assigned?
What tells the IAXy which port to use. The IAXy provisioning file
"iaxy.conf" file does not specify a port (I'm configured for
static IP).
I'm trying to use the IAXy device across a VPN.