With a few simple steps you can use the same wires and same phones that your POTS (Plain Old Telephone Service) use. You can have several phones plugged in and they will all ring when someone calls.
This will not allow you to use 2 phones at the same time, on the same call or even two separate calls. it just allows you to use your "normal" phones on your VOIP service through your existing wiring. And it allows you to use your VOIP service more like normal phones.
Do this at your own risk, you can get shocked or injured by doing this incorrectly. This system works for me and my configuration but it may not work for you. Make sure you understand enough about it and decided if it will work for your situation.
Step 1: Materials
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.
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.