Viber allows you to make free calls from your iPhone to anyone who has the app installed.
Viber works in a similar way to Skype, using VOIP to let you talk for free to other Viber users over 3G or WiFi. It also supports free text messaging, allowing you to send and receive short messages from your Viber friends.
Unlike Skype, you don't need to sign up to Viber to use it. You just register your phone number once, and you'll receive an SMS message with a code. Enter this code into Viber and that's it. You'll never need to sign in again.
All of your phone contacts are automatically imported into the app, and a Viber logo is displayed next to any contact who also has it installed. You can then just click on their name to call them for free - regardless of where in the world they are, or even what phone they are using.
If a contact does not have Viber installed, you can still ring them through the app, although you will get charged at the cost of a regular call.The quality of calls with Viber is generally very good, and rivals Skype in this respect.
We did experience problems receiving calls, during tests on an iPhone 3G. Viber is designed to run in the background, and uses push notifications to display details of incoming calls and play a sound. In our case, however, there was no ringer or message alert when we made a call from another phone. When entering the app, however, details of this missed call were displayed. It's an issue that's also been mentioned from commentors on Viber's App Store page, so hopefully this will be addressed in an update.
The Viber interface is very clearly laid, making it very obvious how to use the app. The program could do with some more features, though, such as conference calling and video calling.
Overall, Viber makes for a simple and effective way to make free iPhone calls.