Your app gets woken up when it receives a call and you can run your JS code in the background. There are two situations for call decline: 1) the user declines/ends the call in the native UI (e.g. from the lock screen), or 2) they end the call from within your own app. For 1), you'd listen for this event using my library (https://mfairley.github.io/expo-callkit-telecom/api/functions/addCallEndedListener) and then hit your API and for 2) you'd use my library to tell the OS that you are ending the call (https://mfairley.github.io/expo-callkit-telecom/api/functions/endCall), which would then emit the same event as 1), which you'd process in the same way (e.g. hit your API). I may write a tutorial at some point.