This is because Viber Out credits are assigned to your phone number, which serves as your Viber ID.
You will receive a receipt for every purchase via e-mail at the end of each successful transaction.
Use Air Cam to remotely monitor them, while you are in another room.
Why limiting to your pets; do it to your roommates too.
Receipts for transactions made through in-app service providers will be sent directly from those providers.
Viber offers an option to have your Viber Out credits renew automatically when your balance falls below .00 USD.