This seems to be a well liked app. There are no suggestions it does any of the things you are concerned about. It simply provides free telephone calls and messaging to other Viber users.
This is an official representative of Viber Media.
I'd like to reassure you - messages won't be sent to people randomly, and We don't send your information your 3rd-party companies, we don't sell it, etc.
If you have any other specific concerns regarding Viber - please feel free.