I took a look into this for you and I couldn't find any instructions on setting up or using this feature. T-Mobile's voicemail system doesn't have a text option so that is most likely why you're not able to get it working.
Voicemail to text is available to all devices. You do have to active visual voicemail before using the voicemail to text option. You can check out Voicemail to Text Overview for more information on this feature. Have you already activated visual voicemail?