you should check section in "Declaring Your App’s Supported Background Task". in link iOS App Programming Guide
Implementing a VoIP App
A Voice over Internet Protocol (VoIP) app allows the user to make phone calls using an Internet connection instead of the device’s cellular service. Such an app needs to maintain a persistent network connection to its associated service so that it can receive incoming calls and other relevant data. Rather than keep VoIP apps awake all the time, the system allows them to be suspended and provides facilities for monitoring their sockets for them. When incoming traffic is detected, the system wakes up the VoIP app and returns control of its sockets to it.
To configure a VoIP app, you must do the following:
Enable support for Voice over IP from the Background modes section of the Capabilities tab in your Xcode project. (You can also enable this support by including the UIBackgroundModes key with the voip value in your app’s Info.plist file.)
Configure one of the app’s sockets for VoIP usage.
Before moving to the background, call the setKeepAliveTimeout:handler: method to install a handler to be executed periodically. Your app can use this handler to maintain its service connection.
Configure your audio session to handle transitions to and from active use.
Including the voip value in the UIBackgroundModes key lets the system know that it should allow the app to run in the background as needed to manage its network sockets. An app with this key is also relaunched in the background immediately after system boot to ensure that the VoIP services are always available.