If you get what is known as a "Handy" adaptor, then you can plug in your ordinary DECT phones to it.
The Handy itself, plugs into your router and then provides a phone socket out. It is a great idea, but you can run into a bit of mire if you end up having to forward ports, or reconfigure network properties. When it works, it is wonderful - but when it doesn't, it stinks. My advice is to make sure that you have a good provider before doing anything.
Conan