VIBER - Free calls iPhone to iPhone
Brilliant little app for making free calls to other iphones.
All calls are made via 3G or wifi if you are connected, and once installed it scans your contacts for other Viber users (same as whatsapp does) and then you can call them for free.
The interface is exactly the same as your iphone's normal telephone app, just in purple.
I've been using it for a few days now and can't fault it.
Give it a go.
Viber - Free Phone Calls for iPhone, iPod touch, and iPad on the iTunes App Store