FaceTime might be easy but network traversal can be hard, which is why Apple has posted a knowledge base article to help you get your FaceTime behind a firewall.
If the Wi-Fi network router that you are connected to uses a firewall or security software to restrict Internet access, contact the network administrator and reference this technical article. To use FaceTime on a restricted Wi-Fi network, port forwarding must be enabled for ports 53, 80, 443, 4080, 5223, and 16393-16472 (UDP).
The Wi-Fi network administrator can refer to their router, firewall, or security software documentation for information on configuring port forwarding.
If you encounter issues using a Wi-Fi network, use standard Wi-Fi network troubleshooting to resolve interference and other issues.
Have you had any firewall issues with FaceTime at home, at school, or at work? If so, did this tip help?