Cross-Platform BLE Call-Reactive App

I want to ship a lean Android + iOS application that can 1) establish a Bluetooth Low Energy link to my peripheral and move data back and forth, and 2) instantly push out a user-defined BLE packet the moment the phone detects an incoming call.

The BLE work is classic data-transfer: discover the device, pair, subscribe to characteristics, then read/write packets on demand. Stability in the background is critical; the link must stay alive (or reconnect quickly) when the screen is off.

When a call arrives the app should listen to the system broadcast (Android) or CallKit/VoIP push workaround (iOS), pull the string the user previously defined, and write it over the existing BLE connection without visible delay.

Please send a concise, detailed project proposal outlining:
• your preferred tech stack (e.g., Kotlin + Swift, Flutter, React Native, etc.) and why it meets both BLE and call-event constraints,
• an architecture plan for background operation on both platforms,
• how you will test timing reliability of the call-triggered message,
• past apps that prove you have already handled BLE data packets or telephony events.

Deliverables:
• Full source code with build instructions
• Debug and release builds for Android (APK/AAB) and iOS (TestFlight)
• A minimal UI to select the peripheral, enter the custom “on-call” message, and view basic connection logs
• Short README documenting characteristic UUIDs, message format, and any platform-specific permissions

I’m ready to move quickly, so the clearer your proposal, the faster we can start.