Off of your comment I found this (applicable for iphone 4), it may work but I am unable to test right now.
"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)." - Apple site