VoIP (Voice over Internet Protocol) is the ability to use the internet or your local network as a telephone. If you’re using Skype, you’re using VoIP! If you mostly want to make calls to or from normal phones, then it’s time to consider a standards based VoIP provider.
Dipping your toe in VoIP waters is easy. Competition is fierce and there are plenty of great providers. To get started, I can recommend OzSite Internet Services, who offer a free starter plan with no contract, 12c untimed calls to fixed phones and 27c/minute to mobiles. You sign up, buy some credit (it works like a prepaid mobile), install free softphone software, and you can talk to Aunt Edna in Oodnadatta for 4 hours for a flat 12c. 
If using your PC and headset becomes a nuisance, the money you have saved on phone calls will soon buy a SIP Phone. This is a phone that has a network connector just like your computer, instead of a phone line connector, and plugs straight into your network hub. It replaces the softphone and means your PC doesn’t need to be on for you to stay in touch.
If you really like your current regular handset or need to hook in a complete office PABX then you need an ATA (Analog Telephone Adaptor). This is the little black box with a network port on 1 side and phone “exchange line” socket on the other that connects existing hardware to VoIP. 
Once making calls is under control it’s time to get a DID so people can call you. A DID (Direct InDial) number is a normal 10-digit phone number that can be connected to your VoIP service. DIDs are available for all capital cities and most regional areas so your callers dial a local number. You can even port an existing number to use as a DID.
The VoIP space is hotly contested. Recently I have seen on offer $4/mo/DID, no contract, 9c/call un-timed Australia-wide, 25c/min to mobiles (no flagfall), 5c/min overseas. So for a typical office with 3 lines and a fax number, doing a quick calc, I figure a total of $27/mo ALL UP! Voicemail and call diversion is available free; fax is gateway’d and arrives as email.
