If you are looking for a phone to use just for your VoIP phone line, then you may want to consider a USB phone. And with a SIP provider you can call and recieve calls from traditional land line telephones through your VoIP phone. Here is a link for USB phones:
http://www.altoedge.com/usbphones/index.html

If you want to be able to plug your regular phone into your computer, then what you need is a telephony board for your computer as opposed to a certain phone. You can find telephony boards at the link below, but if you are going to make the switch why not go all the way and just transfer your phone number to a SIP provider.
http://www.altoedge.com/telephony/index.html