To get facetime working on your firewall you need to be sure some ports can be used. For most home users this won’t be a problem but it may be different at work. Here is the Apple KB Article on it :
http://support.apple.com/kb/HT4245
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 443 (TCP), 3478–3497 (UDP), 16384–16386 (UDP), and 16393–16402 (UDP).
Make sure those UDP port ranges have a good priority in your QoS configuration and you should be good to go. It is worth noting that DNS and HTTP must be open to the outside as well, but they are probably used only to establish the call (same for HTTPS/443) so the QoS config should not matter.
