你的公司或者教育机构可能使用防火墙来增强安全性.防火墙通过阻止特定的internet 流量进出来工作.
Internet 流量 通过防火墙时基于服务id的数字,这也叫端口.

Internet traffic moves through a firewall based on service-identification numbers that are referred to as ports. Certain ports must be open for FaceTime to work. Network administrators typically open a minimal set of network ports, allowing the traffic for approved applications to enter and leave the network while blocking other network traffic.

Ports to open

If the network router that you are connected to uses a firewall or security software to restrict Internet access, contact the network administrator and refer them to this article.

To use FaceTime on a restricted network, port forwarding must be enabled for these ports:

443 (TCP)
3478 through 3497 (UDP)
5223 (TCP)
16384 through 16387 (UDP)
16393 through 16402 (UDP)
Depending on the NAT configuration for the router and network, additional ports may be used to send and receive video. Some router-specific features or configurations may interfere with FaceTime. This includes port mapping on either end, SIP dropping, or dynamic opening of media ports.

The network administrator can refer to their router, firewall, or security software documentation for information about configuring port forwarding.

Additional Information
If you encounter issues using a Wi-Fi network, use standard Wi-Fi network troubleshooting to resolve interference and other issues.

FaceTime for Mac support resources can be found here. The support page has links to support pages for FaceTime on iOS devices, on the left side.
http://support.apple.com/kb/HT4245