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(...);