I also had this problem and tried a number of options.
Port forwarding did not do the trick and as it poses a security risk it`s not the wisest thing to do anyway.
But after a very long search I found another tip which was to set the DNS of every Apple device you want to run FaceTime on,
I tried this,
and now FaceTime works on my iPad2 wifi/32 and also on my iMac and my MacBook Pro.
On the same wireless network!
So this action seems to solve both the "can`t connect" problem and the problem of not being able to run FaceTime at the same time on multiple Macs.
Just to make sure you should write down the original DNS address which is probably 10.0.0.138..
then go to settings,
find the wireless network you`re using,
and tap the blue arrow thing next to the network name.
There you will be able to access the DNS number.
Just change it and save settings,
then start FaceTime again.
Hope this helps guys.
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