Right now, without VoIP API, if you get a cellular phone call while using a VoIP app, your call gets put on hold/dropped/whatever because the cellular call takes precedence. What will the behavior be like after apps update using the VoIP API?
I recommend getting the WWDC app and watching the video on it. Pretty sure you get UI for this to put calls on hold/reject/end whilst you take another and so on.
I can summarise it like this: VoIP calls will be visually and functionality indistinguishable from regular calls and FaceTime calls. They use the same UI and adopt most of the same behaviour. When you receive an actual call while you are VoIPing using this API, then you can deal with it in the same way: switch to the call or put in on hold. They are also integrated in the phone app and contacts, Siri, Bluetooth and the do-not-disturb mode.
No because Skype for business has not updated for iOS 10. So they are not taking advantage of the new APIs. Nor will they be able to until it is released.
I can summarise it like this: VoIP calls will be visually and functionality indistinguishable from regular calls and FaceTime calls. They use the same UI and adopt most of the same behaviour. When you receive an actual call while you are VoIPing using this API, then you can deal with it in the same way: switch to the call or put in on hold. They are also integrated in the phone app and contacts, Siri, Bluetooth and the do-not-disturb mode.
You also get the green banner at the top to return to the call when you go out of the call with the home button and so on.
One thing I wasn't clear on (maybe I need to rewatch it or look over documentation) but in their examples it seemed that although receiving a VoIP call would present you with the native phone UI, when you initiate a VoIP call from favourites, recents or contacts, it will open the app in question- thus possibly only presenting the app's custom call UI. But perhaps even in this case the native UI is presented in-app.