You don’t really need to do that when the app goes to the background. On both iOS and Android you’ll be allowed to run on the background, as long as you have defined your background permissions right (iOS).
On Android >= 8 you app may still get killed, so make sure you use a foreground service while in a call: https://github.com/voximplant/react-native-foreground-service
Note that in the background JS timers don’t work, you can monkey patch setTimeout and friends to use: https://github.com/ocetnik/react-native-background-timer
Cheers!