Viber allow you to make Free Calls and SMSs to other Viber registered Mobile number anywhere in the world. How Viber is different from other VoIP Instant Messaging Clients is that it uses you Mobile Number as unique user ID and by accessing you contact list it can tell you other Viber users. And best of all Viber is a free App from Appstore. Viber is compatible with iPhone 4, iPhone 3GS, iPhone 3G, iPad 2, iPad and iPod Touch.
Facetime provides similar type of functionality alongwith Video calls but its restricted for use in authorized countries, for the rest its a cumbersome procedure to setup. Moreover Facetime is only available for iPhone 4 to iPhone 4 users which is bottleneck as Apple can not force everyone to buy its product to use Facetime. Therefore Viber as recently opened its restriction of Apple only and is also available for Android users also [link].
This application is absolutely free and it recognizes already registered viber users from your contact list. To install simply download the free app from Appstore on your iPhone. Appstore requires an apple account to download free application so if you want to setup Appstore account without credit card visit here
To install Viber simply search for Viber in Appstore and download/install. Setting up Viber is simple as it asks for you mobile number and sends a verification code via SMS. You just have to enter that code and you Viber App is all set to go.
You can click Viber in Contacts to see your friends who are already using Viber.
Viber also provides you a decent text messaging option under messages tab using conversation style texting. And surprisingly the call setup time is superb as compared to conventional Carrier based call and other instant messaging clients.
I would personally recommend viber to all iPhone users and especially iPad and iPod users who do not have any call facility available on their devices. Moreover its cross-platform application that is Android users can also join the party.
View Viber at iTunes from here