The best way to learn VoIP is by experiencing it. Try to buy small IP PBX or some wireless IP PBX. You will be surprised that it was very very easy to learn even through self-learning. All you need is manual of it, then experiment on its features. when you are stack on an unsolve trouble, just reset. hehehe..
you can learn by online tutorials, but it wouldn’t last to your memory since you didn’t really experience it.. you can also experiment with Skype, or start with Yahoo messenger, or google talk.. there are lot of ways to easily study Voip.. but experiencing it is the most effective and fastest.