This miniature, 8-line home telephone exchange (PABX) works with standard telephones and features high-quality, user-friendly progress tones. Connection to telephones is via a two-wire unbalanced loop with a common ground. Only pulse dialling is supported although the exchange could be adapted to use a DTMF tone decoder.
http://www.holmea.demon.co.uk/Exchange/Design.htm
I heard about the Sipura and started looking around on how to use it, and then I heard about VoIPBuster which provided free VoIP calls to land lines all over the world. I eventually figured out how to actually get a user friendly, transparent, phone solution up running and now anyone in my house can pick up any phone and the Sipura takes care of routing the call the cheapest way.
http://www.grynx.com/projects/voip-for-free-with-the-sipura/