Viber out account is blocked only if some find suspected action against the terms of services given by the Viber. The person may not able to use the Viber if he performs irrelevant activities or send some suspicious messages.
Moreover, viber out can be blocked if a person does not have a recharged account for a long period of time.
In case, if the Viber out account is blocked and you want to unblock it then open the support ticket. Here you can provide the proof and get activation of your account.