See Google Talk and Skype, both of which can do voice-only calls and video calls over 3G. It's not all perfect, though...

The problem with Skype on a mobile is that keeping a connection to their P2P network requires much more power than the usual push notification channel does. It's more expensive computationally and traffic-wise.

Google Talk, in comparison, needs less power, but the audio and video quality is just much worse (on my Nexus S with ICS). I also have fewer contacts in GTalk than in Skype. And for some reason, on many phones, there is no audio and video support at all.