Yes, you can use the Yahoo! Voice service through Yahoo! Messenger with Voice from behind a firewall. We do not recommend using it from a wall that is on fire though. That can get you in a hot position you might not want to be in.

For most home editions of firewall software, you'll need to open all outgoing TCP ports in the range of 5000 to 65535 (so Yahoo! Messenger can make PC-to-PC calls).

Yahoo! Messenger with Voice will select a random port in the 5050-5070 range to set up the call, and then it selects a random port in the 5000-65535 to pass the call media. If you only have access to ports 80 (HTTP) and 443 (HTTPS), Yahoo! Messenger's PC-to-PC calls won't work. Learn more about which ports Yahoo! Messenger uses.

If you're behind a corporate firewall, please contact your network administrator for assistance getting around this issue.