How to implement Callkeep with Agora in Flutter?
For IOS you will have to implement a server side script that will send VOIP Push notifications to the user before the real VOIP call comes in. From what I know, you cannot user Firebase for this.
Please refer to https://developer.apple.com/documentation/pushkit/responding_to_voip_notifications_from_pushkit to know more about the changes about handling VOIP calls in background.
Also, refer to https://developer.apple.com/documentation/pushkit/responding_to_voip_notifications_from_pushkit to know more about sending VOIP Push notifications on server side.