Can't connect to socket server though flash client

Question

I have a web app built in Flex which connects to a socket server on my server. However, some users are stating that they're unable to connect to the server.

I was wondering, what ports (on the clients machine) would need to be opened to connect to a socket server through Flash?

Everything I've found has been sending me to Socket.io and WebSockets or talking about the Flash Policy File requests on the server...

Basically, the majority of users are able to connect, but something is stopping a few of them and I have a feeling it's their firewall configuration. However, I don't know what to tell them to configure to make it work properly.