I am testing a Voip client clip written by an iphone app developer who uses PJSip. I personally was involved in the development of SIP myself, but not an expert.
With the help of our VOIP softswitch (Acme / Broadsoft package), he takes care of all Nat processing methods, dynamically determining the best re-registration time in order to save Nat's live spot on the nat device. Usually, when the iphone is behind NAT, this time is 60-90 seconds.
Iphone uses TCP and is happy to work with Backgrounding. Obviously, the background image is ideal for saving battery power. A call wakes up the phone and you can listen to the call.
The problem is that when the application goes into the background, it has no way to open the Nat Pinhole postcard. This means that the firewall will always reject new incoming calls from accessing the Iphone application after closing the hole. This affects incoming calls running in the application for several minutes, and then does not work for a certain period of time, and then when the application leaves the background and registers again, incoming calls work again.
According to the apple documentation:
http://developer.apple.com/library/ios/#documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/ManagingYourApplicationsFlow/ManagingYourApplicationsFlow.html#//apple_ref/doc/uid/TP40007072-CH4-W4WW
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