VOIP stands for Voice Over Internet Protocol. Which means nothing to most people. What it really means, is that you can make phone calls using the internet. This can cut your phone costs, especially international phone costs.
Most ISP's have either a VOIP service or one that they can recommend to you. If you want all your desk phones to run on VOIP, then consult a phone specialist or your ISP to work out how to do this. In most cases, this is an expensive option, especially if you have a phone system that is working OK, but if you are looking at getting a phone system for some reason, consider making sure that it is VOIP compatible. You may be able to just get an add on that will allow your existing phone system to use VOIP.
The cheapest and simplest option is to use Skype. Just download it to your PC, install and if the other party has Skype as well, phone calls are now free anywhere in the world. You do need some way to talk and listen, so you will need a microphone and earphones or headphones. Many laptops have these built in, but the quality isn't brilliant. You can generally get a mic/headphone set from under $20. Check first that your PC has a sound card or get a USB mic/headphone. You can also use video to have a video conference (if you have a webcam).
If you need to phone to a desk phone somewhere, there is Skype Out. You simply pre-purchase the ability to phone out via Skype and you can then call any phone anywhere in the world for a fraction of the price of using Telstra or other carrier. And we do mean fraction of the price. We've seen examples where it has been around 100 times or more cheaper.
You can also have a Skype number where people can phone from their normal phone to you on Skype. You guessed it, it's called Skype In. They must have very few marketing people at Skype. There are alternatives to Skype, but we'd recommend sticking with something that other people are likely to be using and is fairly well understood.