Hi Dion,
The port you are seeing is the port used by the devices (phones/softphones/...) to send SIP messages to your server.
I guess your phones are located behind a router on a local network. When the first phone sends SIP messages to your FS, it leaves the router with the port 5060 (it's up to the router to re-use 5060 or chose a random port from the beginning) but then for the other phones it has to map to other external ports, that's why a random one is used.
Your router might be able to provide a way to configure NAT mappings manually, you should look that way.