If you use FaceTime and iMessage behind a firewall
You can use FaceTime and iMessage with most networks. If you're on a network that's behind a firewall, you might need to enable certain ports.
Some networks use firewalls for security. Firewalls block certain Internet traffic from entering or leaving a network. Internet traffic moves through a firewall using ports. To use FaceTime and iMessage behind certain firewalls, you might need to ask your network administrator to enable these ports:
Depending on the NAT configuration of your router and network, additional ports might be used to send and receive video. Some router features, such as port mapping, SIP dropping, or dynamic opening of media ports might interfere with FaceTime and iMessage. Contact your network administrator if you need help with port forwarding.
If you're using a Wi-Fi network, learn about recommended settings for Wi-Fi routers and access points.