Have a normal phone you want to make into a USB voip phone?  An RJ11 to USB adapter will let you connect any old standard phone that uses RJ11 (the standard phone line) and turn it into a voip phone.

Build a Your Own Voip Phone
GRYNX wrote a three page guide that shows how you can convert an old wireless phone into a voip phone for your computer.  However, this will required soldering wires to the board of your phone.  It also doesn’t use USB, rather it uses the audio jacks of your computer to connect your phone to your pc.

Build Your Own RJ11 to USB Adapter
This guide is a bit difficult to follow if you don’t understand wiring diagrams.  After looking at the pics, I’m not actually sure I would attempt this project myself. If you want to give it a look just view this guide.

Buy a RJ11 to USB adapter
If all these guides seemed too complicated for you, you can buy your own for about $32.  This store sells them, but you could also try ebay.