If you are using Viber for the first time, you need to set it up and activate it with your phone number. To activate Viber for Android phones and tablets do as follows:
1. Open "Viber"
2. A greeting page is shown, tap "Continue"
3. Now the App asks for a permission to access your Address Book to sync your contact list. (you must allow this permission)
4. Your country is shown and you need to enter your phone number. (You may change the country)
5. Now, confirm your phone number and wait for a text message which includes the activation code
6. Enter the code to continue. (On some devices, the code is entered automatically as soon as the message is received)
7. Now, enjoy using Viber!
** If you didn't get a SMS after a while, a "PRESS HERE TO GET A CALL" button appears. Tap it and a page is opened in the browser. Confirm or change your number and tap "Get Code" and wait for a call from Viber
See Also: How to Deactivate Viber Account on Android