not correct at all - nc can also use udp/4500 for ipsec instead of ssl.
securemeeting only uses 443, right. BUT it problems with some proxyservers can occur, had that lots of times too.
There is no other specific ports to be open, to access secure meeting.
Can you add you internal ip url, to the bypass proxy list on the browser and check if that helps you.
Else, please go through the debug log from the user machine, which will let you know where it is breaking.