Free VOIP?

Does anyone know of a completely free VoIP app for the iPod Touch? I have seen truphone for ipod, but it is only free to call other truphone and Google Talk users. I would like to get an app that lets you call any number in the CONUS free. Is there any such app at this point?

What about Fring?

I think this app. should be fine
Search it in App. store it's free

I have downloaded that app, but I'm not sure if I know how to use it, or if it isn't what I need. From what I can tell, you need some other service to use it. Skype is one of the options it lists, but they charge for landline and mobile calls, which is what I am trying to do for free. Another option listed is SIP. What is that, and are there any free providers? My main problem seems to be that there is cost involved somewhere no matter what I try to do. Is what i am trying to do impossible at this point?

Sorry for all of my questions and thanks for any help you guys provide!