I need robust, production-ready call-style notifications added to my React Native consultation app. The system has to deliver incoming-call pop-ups, missed-call notices and general alerts on both iOS and Android, no matter whether the app is in the foreground, running in the background, or completely killed. My stack is already in place—React Native on the client, with an Express.js + MongoDB backend. What’s missing is the native wiring and API logic that makes the whole flow reliable: • iOS: APNs VoIP / CallKit integration so an incoming call looks and behaves like a real phone call. • Android: FCM push with ConnectionService (or a proven alternative) to surface full-screen call UIs. • Server: endpoints that craft the right payloads, manage tokens and handle delivery receipts. Acceptance is straightforward: notifications arrive within seconds, wake the device in every app state, route the user into the correct screen inside the app, and record the event in MongoDB. If you have shipped something similar, show me a quick sample or link to past work and we can move fast.