They are:

• 5050
• 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).

If your connection to the web is through an http proxy, then you won't be able to make or receive calls with your PC.

Don't know if you're using a proxy? You can check by following these steps:

Click Messenger in the upper left corner of your Messenger window and select Preferences.

From the Category list at left, choose Connection.

Which button is selected? This will tell you your proxy status. The text below the buttons explains which button is most appropriate for your user environment. If you’re behind a corporate firewall, please contact your network administrator for assistance getting around this issue.