VoIP or Voice over IP is growing rapidly in internet world. The main use of it is in making free internet phone calls through VoIP. Also there are many handsets coming up in the market which are specially meant for providing VoIP call service. Such handsets looks similar to a mobile phone and can also connect through Wi-Fi, so double advantage - Free calls on move

Also many free VoIP softwares available for all operating systems. This will enable you to make PC-to-PC calls to your friends when you are connected. Some of such softwares are available from Skype and Gizmo websites. Skype also offers PC to phone calls if you buy some credits.

To make such PC to phone calls, a network has to install a VoIP gateway or a PBX gateway and then connect them to your PSTN device which will indirectly make such calls paid and so service providers charge for them.
Some of such Paid PC to phone VoIP calls service provicers are Vonage, AT & T Call Vantage, Lingo, Skype etc.