If you delete the Viber application from your iPhone, or, if your account is inactive for any other reason, your address book data will be removed from Viber servers after 45 days. However, the good news is your phone number (account) will be removed only after 1 year of inactivity.
Viber 2.0 has a “Deactivate Account” option under a “More” tab that instantly deactivates your account. By choosing to deactivate your account in this manner, you will be deleted from the list of Viber contacts on your friends’ phones.
It is good to know, although you have never explicitly created a Viber account as such, it will be created as soon as you have typed in your phone number during the process of application activation. In short, your phone number is your account. Your address book data and your call logs are stored and associated together with it.