You need a VoIP service that have supported phone adapters Skype and Vonage are some of the most well known.
A VoIP Phone Adapter. http://shop.ebay.com/i.html?_nkw=USB+VOIP+phone+adapter&_sacat=0&_trksid=p3286.m270.l1313&_odkw=USB+VOIP+adapter&_osacat=0
(2) Leviton Voice Grade 4-Conductor QuickPort connectors. You can use 6 or 8 conductor ports as well.
(1) Leviton QuickPort Single-Gang Wall plates (a 2 location cover plate)
If you have an older home you might need to buy a single gang electrical box.Tools:
Flat screw driver and
Wire insert tool for the quick connect adapters.
A volt meter will come in handy for testing "live" lines but isn't necessary but recommended.