Object Oriented Program Design Questions

It sounds like you want to use a custom EventListener object.

From the Phone class, let's say you want there to be a ringing event that displays in the main window. Create an interface like this:

public interface RingListener {    
    public void ringing();
}

In the Phone class, create a method call addRingListener(RingListener listener) and store listeners added in a List, such as an ArrayList.

In the MainWindow class, store an instance of your Phone object and add a RingListener to it, implementing the abstract ringing method. Then type in your code for what happens.

phone.addRingListener(new RingListener() {

    @Override
    public void ringing() {
        //Do stuff
    }
}

In the Phone class, whenever the event happens, iterate through all of the RingListeners in your List and call the ringing method.

//Ringing happens!
for (RingListener listener : ringListeners) {
    listener.ringing();
}

This is just a quick overview of how the EventListener ideology works. I'd suggest taking a look at ActionListener and see how it works, as well as searching for some tutorials on how to create your own custom listeners.

1. That's probably because you have all the classes in a single package and you are using no access modifiers and thus making all members package-protected (perhaps package-public is a better name). Use access modifier private for member variables and/or separate classes in different packages more logically.

2. If I understand your design correctly, shouldn't you be using class PhoneMessenger to deliver events? In this case, don't think in terms of "update a value in the MainWindow". Think in terms "an event X happened in the phone and I'm going to notify interested parties (MainWindow)". When MainWindow receives this event, it is its responsibility to decide what to do (change the value of the variable in this case), not class Phone.

First, you need to aware about OPEN CLOSE PRINCIPLE. This is for oo design approach. and try to use interface which limits the scope of any class with defined methods. To access any object's value, always go for a POJO class. Here are the links which you needed how to design efficiently.
1.Open Close Principle
2. Interface Segregation Principle
3.other design principles

Have a look at my question. I was facing a similiar problem. However the users of SO are very smart. Have a look at this --> Print transaction status on TextArea