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 for PC-to-PC voice conversations, Yahoo! Phone Out, and Yahoo! Phone In to work.

If your connection to the web is through an HTTP proxy, then unfortunately you won't be able to use Yahoo! Messenger’s voice services.

Don't know if you're using a proxy (or even what the heck one is)? You can check by following these steps:

1. From Yahoo! Messenger’s main menu, select Preferences.
2. Choose Connection from the list of categories on the left.
3. If “Use HTTP” and “Use an HTTP proxy” are checked, you’re connecting through an HTTP proxy server. If you’re behind a corporate firewall, please contact your network administrator for assistance getting around this issue.