Today, Mobile VOIP uses wi-fi, 2G/3G data channel but with 4G which is end to end IP, all that will soon change and mobile operators will see huge drop in their voice volume.
Gartner Research says "Mobile VOIP poses a huge and direct challenge to the $690+ billion global mobile voice market."
Making costly international call from your mobile will soon become thing of past.
Let's see the few best options for Mobile VOIP.
SKYPE : One of the very popular application. However it doesn't mean the best as currently it only supports wi-fi and not 3G data channel.
VOPIUM : It works with both wi-fi and 3G and it is compatible with Java, symbian, Blackberry, Windows Mobile, iPhone, Android, etc devices.
All these apps are simple and easy to use and cut down cost on your international calls.
My recommendations : Truphone and VOPIUM.