You may have a land line phone, a mobile, a computer with say Skype or Ekiga. Sometimes you need to make a phone call somewhere in the world, or indeed make a national call, at a rate less than your generous phone company. If you have a computer or mobile where you can get a speedy broadband connection you can most likely make cheap calls with relative ease.
VOIP (and SIP) are established open protocols to make calls over the internet, and you may well be using them in your communications software already. The details do not matter other than they work.
To make cheap phone calls using the internet you need:
A good internet connection. If you are on the go and can find a wifi hotspot you are good. If you have a broadband connection at home you are good.
A VOIP provider.
Some hardware (which you may already have) and maybe some free software.
Once those elements are in place you are good to go.
I use a US company called Callcentric to make phone calls. I use the pay-as-you-go service and started off with the minimum $5 to test it out. It worked for me so I keep topping up the account.
You can check to see how much it costs to make a phone call to various countries here. Landlines and mobiles have different rates:
There is a wide range of supported devices which you can use. Adaptors plumb into your network and you plug an ordinary phone into them. Softphones are programmes which run on desktops or certain mobile phones. You can also choose IP phones which plumb into your network and work just like ordinary phones. You can also use cordless phones. Your choice.
Once you have signed up and have a device or software installed the next stage is to configure and test your phone. The instructions for each device can be seen in the Callcentric support pages. It is a bit of a fiddle to get them set up. If you stick to their online guide it is relatively painless. Once you have done this it is time to test the phone, after which you have another way to make phone calls.