I have been researching this for a while...
Apple would bascially say: "No".
A lot of people that do enterprise secured VOIP are crying on this forum and it looks like Apple doesn't care.
The current work around is to build against iOS 9 SDK with XCode 7. But there is no future to that.
Our app suffers from the same problem. Luckily most of our customer that need closed network use Android. Apple would probably lost a few sales over there just because of this...
Thank you for your answer. Did apple officialy confirmed that these 2 ways that I have mentioned is not gonna work at that you say Apple basically says: "No." to that without showing an alternative solution to it? The business is a rational and very possible solution and I think apple should have a rational answer to the accured problem. If not we have the android app ready and working in that organization and we should abandon everyone with an iPhone and that is not a good way to do that.
See this thread: https://forums.developer.apple.com/thread/50106
Our app use to keep socket open in background, but in iOS10 all the socket will be killed after a couple of seconds when you enter the background.
If your app doesn't need to go to AppStore, you might get away with faking a APNS server in your local network, but that is really a long shot...