I always thinking about to have a home-brewed VOIP server. Such as this one, Build a Skype Server for Your Home Phone System, it is a skype server, not exactly what I want, but still good.
Through this Skype server, you can have a small phone system in your home.
The full function of home VOIP server is based on Open-source software and free of charge.
With two or three VOIP card, you will have a system, you can call out, or receive incoming call.
You can also have a conference call in your home.
Do you need it?
Running on Linux.