The implementation of functionality when an application in the background more or terminated/killed state is implemented in our WebRTC samples.
You can find Quick Start for video calling samples in the corresponding section for each SDK.
Be notified that you need to configure push notifications for your application/sample: Configuration of chat and video push notifications
While integrating the functionality from the sample, you need to configure push notifications as it is described in the guide as well.
If you have any additional questions, feel free to submit a ticket to our Support team and we will help with all your efforts.