Services or apps like Big Voize don't allow you to register an account. Instead, you have to buy a pre-paid card in order to log-in with an operator code, ID, and password. The information can be obtained from local re-sellers in your country (available in certain regions only). You can also contact the developer of the app using the following email:
firstname.lastname@example.org He should tell you where to buy those pre-paid cards.