I have this setup at home in the UK, but it involves a series of systems that pretty much covers any telephone (and most external internet) facilities, including voicemail, fax, and dial up. My system uses Skype for long distance calling (our family is Skyped) and reduces telephone charges around £10 a quarter. Before we had calling cards, but our telephone provider is sneaky and started to charge us for 0800. I have a mobile and I do not use the minutes up properly (like 400 minutes a month goes to waste) so we looked at only getting a mobile only scenario.
I have a Skype to BT adapter and with works on a Win 9X legacy system with Super Voice. Its an old IBM 240 laptop with built-in modem and works a treat. Its super quiet and if the power fails we have a limited UPS. Because of the driver issues I have it running on a Nlited 9X box with a but I am about to revise the system with Puppy Linux and Asterisk. I was looking at the Raspberry Pi to migrate into.
Its all transparent, no more sales calls at dinner-time (yes they are now doing that crap in the UK), cold calling, and wrong numbers. Most of my calls gets diverted to email notification so I can call them back on my mobile. Nobody really knows if they are calling a land line or via the internet. Our phone bills have gone down 30% and with the parts I had around, it has saved me a bunch of cash. No it doesn't play Mame...yet.
It is a real pain in the butt to get working originally, and finding the drivers for the interfaces was a real challenge, but I wish we had a system like ooma in the UK that I could just get working without recreating mission control.
Also I voice record all incoming and outgoing 0845 numbers to service centers, and I have managed to snag a few dishonest call center staff, so in all a very handy system. I also have non geographical numbers stored for banks and the utility companies so I do not get smacked with additional charges as they keep you on hold for 20 minutes plus. I proxy all my home internet via wifi and with Netlimiter, so SKY+ doesn't hog all the bandwidth.
The best thing about the 9x box, is the speed. It flies in 256mb.
My next project is to move away from TCP/IP and back toa 4.1 Novell Box with IPX for the home wireless network, the fun part is trying to make the SKY router into a bridge.