iOS 10 CallKit

In iOS 10, Callkit gives permission to integrate VOIP (voice over internet protocol) apps with the iPhone user interface, therefore user can give answer of VOIP app on iPhone lock screen as well as view contacts of VOIP app in the phone app’s favorites and recent views. 

Apple has given a SpeakerBox Example app as a reference to implement this, which is here

https://developer.apple.com/library/prerelease/content/samplecode/Speakerbox/Introduction/Intro.html#//apple_ref/doc/uid/TP40017290-Intro-DontLinkElementID_2

references

http://www.howtoisolve.com/apple-ios-10-sdk-api-services/