NetMeeting under Win98 can't call Microsoft Servers

I have a server running Windows 2000 and another server running
Windows 2003. I can't call these servers using NetMeeting. It says "The other
person doesn't allow you to join the meeting." Under
Tools->Options->Security->General->Outgoing Calls, I ticked only "I prefer to
make secure outgoing calls".
There is nothing wrong with the servers as I can call them using
WinXP. So my conclusion is that under Win98, NetMeeting has difficulty
contacting the servers. What I would like to know is that what kind of
difficulties are causing this problem and what I can do about it? Your
feedbacks appreciated.
--
Daniel tan