We are Broadvoice users some years ago, using Unlimited World PLUS product that seems reasonably acceptable.
The problem is that we review Broadvoice config updates a lot of times to make our changes, but they are adding a lot of new countries to the flat rate (which is what alone is all that you want to use to avoid overruns), but the instructions installation are not updated on their website.
It seems that they do in order to use the destinations to which there is no flat rate billing for the additional traffic minutes.
That is why we have proceeded to update the Dialplan so they can make use of the new destinations as a flat fee. We will verify the destinations which are not allowed cell phones so they can improve and optimize their Asterisk Dialplan.
Hope that you help you
Alex Celi Mariátegui