Whenever you want to be able to find out the Caller ID or the number being called during the progress of a call you can use this example:
var subscription = rcsdk.getSubscription();
subscription.on(subscription.events.notification, function(msg) {
console.log(msg.body.activeCalls[n].from); // activeCalls is array
console.log(msg.body.activeCalls[n].to);
})
.register({
events: ['/account/~/extension/~/presence?detailedTelephonyState=true'],
})
.then(...);