How to show ringing UI on mobile even if the app is killed and phone is locked?

I don't exactly know the flow of Video calling in mobile apps. The flow which I am using for video calling is as follows:

1. User 1 taps to call User 2
2. Your app generates a unique name for a Room. Perhaps a combination of the two user's IDs.
3. User 1 connects to Room with generated unique name
4. Using FCM or GCM, send message of high priority with the unique name of the Room and token to User 2
5. User 2 to receives notification and you display a ringing UI(even when the app is killed and phone is locked)

6. User 2 accepts and connects to the same Room

   Now I am stuck on point 5. I am sending the FCM notification(which shows up in tray (edit: now using voip notif) to user2 and now I need to show ringing UI on user2's mobile even if the app is killed and phone is locked. How can I achieve this?