Ok these are the steps I did to setup visual voice mail on my t-mobile blackberry curve 8900
Have the latest 5.0 OS or any 5.0 OS on your BB with the visual voicemail Icon i got it on the net .348
Goto your my.tmobile.com page and change your device to the new 9700 even though your using the 8900
Goto change services and you will see the visual voice mail there for free! Select it and save your plan!
Step 4: resend service books if you have to or just go and enable it on your BB and it should start instantly like it did for me!
Here are my screenshots sorry it took so long for these.
also I have the unlimited data plan for 25 bucks I see they have another one that says 30 dollars when it has the same features as my plan funny how that is...
and the visual voicemail is BEAUTIFUL!