the author of this app says:
pfingo helps you save up to 90% on international calls to landlines and mobiles.
*Great value call rates from 2c/min
*No connection fees, unlike Skype etc
*Works on 3G/WiFi
Suitable for Samsung Galaxy S and Beam, LG Optimus P500, Motorola XT7200, HTC Desire Min. OS1.5 Your mobile operator may charge you for 3G data use.