Using FaceTime and iMessage behind a firewall
iMessage has a sirius flaw : messages shown as deliverd can dissapear in a firewall so that the receiver never gets it - but worse: nobody is not notified about the delivery failure.
For most networks configurations, FaceTime video calling and iMessage will just work. If you are on a network that is behind a firewall, see below.
iPad 2 Wi-Fi + 3G, iPhone 4, iPhone 4S, iPod touch (4th generation), FaceTime
Note: FaceTime and iMessage may not be available in all countries or regions.
Your corporation or educational institution may use a firewall for increased security. Firewalls work by blocking certain Internet traffic from entering or leaving a network.
Internet traffic moves through a firewall based on service-identification numbers that are referred to as ports. Certain ports must be open for FaceTime and iMessage 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.
Enable port forwarding on the following ports to use FaceTime and iMessage on a restricted network.
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 and iMessage. 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.
-- se forum http://forums.imore.com/iphone-5/258417-imessage-wont-work-over-wifi-onl...