You purchase a SIP Voice Box and configure it for your VOIP, then connect it to your ADSL router, then connect a normal telephone to it and use it as normal.
There are now a considerable amount of providers (if you do a search on the internet). The cost saving comes when the calls (local & long distance) are cheaper than your telephone line provider.
One suppliers discription;
Using SIP technology (session initiation protocol) with a "Voice Box", your calls are directed through the Internet rather than the plain old telephone system. The "Voice Box" can be attached to an analog, cordless or DECT telephone, allowing you to make and receive calls as you normally would.
You would probably use this part of the VOIP family in an office or at your home, when you don't want to talk into a handset on the computer.
Happy Calling!
