There are different solutions for both VOIP and Skype. Some need a PC on, others don't.
As I said, I have a new VOIP:
http://www.voiptalk.org/products/index.html
The solutions tend to fall into the following catagories:
1/ PC Phones/devices (headsets etc) that connect to your PC via USB and need software installed and running on your PC.
2/ Phone adaptors, that connect to your router/hub/switch and allow you to connect a normal phone or cordless base station to.
3/ VOIP/Skype phones.
There are advanteges and disadvantages for each.