Calls to the US and Canada are free, rest of the world are at a discounted rate. You get $1 USD as a bonus upon signing up,. If you did not get that credit, email them and they will add it.
After you register and login, you will be able to send yourself the download link to your BB via SMS (free service)
The app can be configured in such a way that if you make an international call or international SMS, it will automatically launch and send it through the app and not through your carrier's SIM card.
I tested it on an 8520, 8900, 9700, 9800, and 9780 and it works perfectly.