How do LINE and Viber display incoming call screens on WP?

Question

I couldn't manage to find that piece of API which allows apps to alert the user with a custom screen, like what can be seen on an incoming (network) phone call over services like LINE or Viber, which even works ubder the lock screen.

Do they need similar registrations as push notifications?

Answer 1

What those apps are using is probably the VoIP APIs that are available. There is a full guide on how to implement such functionality here (including that incoming call screen): http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj206983(v=vs.105).aspx

There is also a downloadable sample app available here: http://code.msdn.microsoft.com/wpapps/ChatterBox-VoIP-sample-app-b1e63b8b