You can easily get Visual Voicemail provisioned without having to call T-Mobile Customer Service.
1. Log into your T-Mobile Account
2. Go here: https://my.t-mobile.com/Plan/OptionalServices.aspx
3. Change your plan from $24.99 G1 Service to $24.99 T-Mobile Android Unlimited Web
4. Submit - You will receive a text confirmation
You get:
Unlimited Internet
Unlimited Text Messages (Up from 400)
Visual Voicemail
You lose:
Nothing! And your bill stays the same.