If you and your friend, relatives uses either android, iphone, blackberry then you can enjoy the services of viber. The first thing that I found interesting is that you do not need to register or sign in when using the service.
Once you download the app to your device, you are asked to enter your phone number and you are given an access code through SMS, which you type in on activation. You are then identified through your mobile phone number on the huge user-base of Viber. Group messaging is another thing that I found interesting, but many other apps have preceded Viber in this.
The app allows you to select and add contacts from your address book for participation. The app integrates your phone’s address book and each time you decide to make a call or send a text message to a contact, you are prompted to either place a regular call(or SMS) through your mobile carrier to the contact or to make the call or send the message using Viber. Before initiating anything in the event Viber is selected, the number is verified to see if it is registered with Viber, as free service is allowed only to those.