Custom Flutter Call Notification Implementation

I need a custom implementation of call notifications for my Flutter app, similar to the functionality provided by flutter_callkit_incoming, but without depending on that library.

Key Requirements:
- Must work on Android
- Integrate with existing Firebase notifications
- Show call notifications when the app is in the background, locked, or killed
- Provide handlers for:
- Accept
- Reject
- Timeout response callbacks to Flutter

Ideal Skills and Experience:
- Experience with Flutter and Firebase
- Familiarity with Android call notification APIs
- Ability to implement background services in Flutter