When installing Viber you are prompted to enter your phone number, which sends a text message containing the activation code to complete the setup of Viber. Sometimes Viber does not send the activation code. If Viber is not sending the activation code, you can click on the No Code or If you do not recieve a code, click here option. This will trigger Viber to call you with an activation code to enter.
In my case, the webpage that initiates a call request would not load. I got around this by waiting for 5 minutes and trying again.