With Visual Voicemail, when you get a voice-mail, T-mobile will send you an audio file that you can play on your handset. That way you don't need to call the voice mail number. The name is a bit of a misonomer as it suggests that you get a transcribed message. You don't.
I've no idea how it works with Google Voice.
I couldn't turn Visual Voicemail on from the the phone either and did it by calling customer service. I think you can do that through My T-Mobile as well. It's free.