I have been using a VOIP system, Skype, for well over a year.
You install the software, and use either a headphone and mic combo or a special phone to talk for free to other Skype users, or to call a landline very cheaply.
To call landlines, you buy credit in advance - pay as you go - and just dial the number using the keyboard or the special Skype phone.
VoIP voice over internet protocol. First is you have to subscribe to voip provider, Then register you existing phone number if you have. Then, you are ready to make calls, your phone system will now rely on internet. Try to read here http://www.pbx101.com.