[Trimix]

Wirelessly posted (L'neuf trois)

Hmmm that's definitely weird. Didn't think the app would work without some sort of data plan (even a generic plan works). As you are on pre-paid they might do things a bit different but I'm not familiar with how Tmobile's pre-paid plans work in general. I would be careful that they are not charging you per kb or per mb of data. That could get expensive. Best way is to call their CS and find out. Better to find out when you owe $10 rather than $100.

As for GV voicemails, I doubt the voicemails are stored on your BB. I think the app preloads the message (buffer?) so the playback is smooth.

As for your last question: haven't tried it myself, but I'm wondering what would happen if you set your Google Voice number as the call forwarding options (Home screen, press call button, menu key, options, call forwarding).