Now that you have made it to Chapter 4, you should have your extensions all set up, or at least a couple of them. If you are unable to call extension-to-extension at this point, then you will need to go back to Chapter 3 and get that working before going any further.
Once your extensions are working, we can begin exploring call routing also called as call control. When someone calls from the outside world, what do you want to do with the call? How do you want your calls to get to an extension? Unless you want your calls to go directly to an extension, you will need to configure one or more of the following features: