TCP ports open or used?

I am running windows 98SE and trying to use PC to PC calling in Yahoo
messenger 8. As soon as I try to accept a call from someone running XP
with a firewall Yahoo messenger crashes. I crash but not the other
person.
I went to yahoo messenger help and I quotethem:
What ports are used by Yahoo! Voice services?
Yahoo! Messenger will work if any one of these TCP ports are open:
5061
443
80
At least ONE of these ports must be open in order to make PC-to-PC or
PC-to-phone (Phone Out) calls, or to receive calls (Phone In).
Where on do I find if theses ports are open and how could I open one if
I need to?

.

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