iphone voip services allowed?
From a technological standpoint this is possible - Skype makes voice calls just fine over 3G data on my iPhone. However, as far as I know, AT&amp;T and Verizon do not permit you to run phone devices on their network without a phone plan, so it's unlikely that you will be able to do what you're looking for, as least officially.

I have heard that some people have had success transplanting the iPad SIM into an iPhone, but it's not something that's guaranteed to work and AT&amp;T may block its use at some point in the future, if they haven't already.