Skype works great, I use it to call my folks in Ireland from here in the UK. Sound quality is as good or better than a normal wired phone.
You don't actually need a USB phone, just one of those headsets with earphones and a microphone boom which plugs into your sound card. You could also use either a USB headset or the aforementioned phone which would install as a separate sound device on your PC. This is a good solution since the person you're talking to doesn't hear themselves back through your speakers then your microphone. This is very distracting for them! Personally, I got a USB bluetooth dongle for my PC, then use my bluetooth headset from my mobile to talk and listen on Skype. This is great since I don't have to sit in front of the computer to talk and can head off and make a cuppa if I feel like it (which happens quite often since you can talk for HOURS when it's free!).
Yes, you can get a USB phone anywhere,
http://tinyurl.com/8vt5y is one, but quite pricey & fancy, you can also get them direct from the Skype site.