If you are reading this post with your iPhone or Android, just click on this link: www.viber.com/dl and install the application. For Blackberry use the following link: viber.com/blackberry and for Windows Phone use this link: viber.com/windowsphone.
Otherwise, open the App Store on your iPhone, Market on your Android device, BlackBerry App World or Windows Phone Marketplace and search for Viber.
If you are an Android user and you can’t install Viber from the Android Market, it means that your phone (or tablet) is not officially supported. In that case, you can install Viber directly from here: http://helpme.viber.com/index.php/Knowledgebase/Article/View/100/16/cant-download-from-android-market
When you install Viber, the app will ask to access your address book, and after that, it will ask you to enter your mobile phone number. When you enter your number, you’ll receive an SMS with an activation code. Enter it in Viber and that’s it!
Please note that, on Android based phones in some cases, you will not have to enter the activation code manually. It will be automatically inserted into Viber.
If you have two SIM cards on your phone, choose the one which will be associated with your Viber account. Once when Viber is installed and activated, anyone that you call will see it as an incoming call from the number you choose, even if you are using the other SIM.
If your device has no SIM card whatsoever (iPad, iPod Touch, Wi-Fi Only Samsung Galaxy Tab), you can enter the number from your mobile phone and get the activation code from it. Then you can use it on Viber, even if the app is on a different device.
Usually, new users are invited with an invitation SMS sent by their friends.