I have just installed and tested Viber on my iPhone 3GS. Viber is an application for iPhone and Android phones that lets you make free phone calls and send text messages to anyone who also has the application installed.
Or as the Viber developers describe: You can call or text any Viber user, anywhere in the world, for free. Viber integrates seamlessly with your existing address book, has great sound quality and once activated, does not require a PIN, username or any additional "in application" purchase. All Viber features are 100% free!
Therefore as long as you have Wi-Fi or a cheap data plan, you can start doing literally free calls and SMSs..spread the word, as the more Viber users, the more people you can contact for free.