Create an incoming call overlay in iOS

I've already done this on Android, but I need to know how and how to do it on iOS. Having a somewhat final "no, this is not possible", I will help me convince the authorities.

The app I have is a VOIP app, although at this point all incoming call processing is done with simple notifications. I know that if our application would use (more unified communications than just voice so that it can be active and not on a call), I could do it easily ... my application.

Its when the app is not in the foreground that I get stuck.

In the "how to do VOIP" examples, use a voip socket to just add a notification. While APN notifications are not very fast, without immediate feedback from the caller, I can't justify including all the extra code (since we're not using SIP, I'll have to write the socket layer myself to get the notifications a little earlier).

Is it as easy as opening a custom url that will launch my app and show the call ( [[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"myapp://example.com/ringer/call-12321123"]];

)? This would seem to prevent me from returning the user to the application they were using before the call started.