IOS cannot receive an incoming call immediately in background

[spextrumaly]

Hi, I've got the problem with this demo on iOS platform.
When I try to call to another phone in peer to peer method, which is iOS platform, I cannot receive the incoming call right away. It shows null is not an object (evaluating 'this.call.answer'). But if I wait around 5 seconds to see the command CallManager: _incomingCall: application is in the background, incoming call is already reported in AppDelegate in log and receive the call, it's working properly.
Thanks for supporting :)