Any update on this issue?
I am now running an iPhone 6 with iOS 10.
Some additional notes from testing:
1. The app does not crash when the phone is placed in airplane mode.
2. With the app running and phone then placed out of airplane mode, the app will continue to run and eventually time out with an error "Registered failed to SIP: account name.
I did not experience this problem on an iPhone 5 that ran iOS 8. I specifically did not upgrade this phone knowing this was an issue with later versions of iOS.
This is definitely a software issue between Apple and Grandstream. I have not seen any way to enable any kind of logging to see what causes the app to crash.
Grandstream seems to be one of the best softphone apps on the market, so it would be ideal for this issue to be resolved. Unfortunately Grandstream has not shown any interest in addressing it.