Are you using an iPhone or an Android? Apple uses its own Voice Mail system and Android phones typically have the T-Mobile voicemail app installed.
What exactly is broken? Are you unable to dial directly into your voicemail or is an app not working or is this visual voicemail?
I started out trying to make the change on https://my.t-mobile.com after much frustration, I phoned 611 and was told that this change could not be done via the web, that I had to use the app.
I installed the Android app, made the change the app spun and spun - no joy, no change.
I contacted support via Twitter, normally I get good results on Twitter, today, I spent three hours with three or four representatives, at some point I was told that that phone #'s account was locked, so they unlocked the account.
I tried the app again, again no joy. At some point, the last rep reached out to his colleagues and fixed me up with the procedure to make the change on the web.
That led me to https://voicemail.my.t-mobile.com and I was able to make the change after many attempts because there is something wrong with the server hosting that site. You can get to the site, look at your vmails, click on settings then get an Error 500 -- Internal Server Error message.
Eventually I was able to get the task done, but it reminded me of the days back in the mid 1990s when I had mobile Internet that kinda-sorta worked.
I'm a TMO fanboy, but they really need to stop everything and perfect what they have before the next expansion.
Yeeeeeeeeesh. It shouldn't have taken that much running around to get a fix, especially considering most won't be as willing to explore and well versed as you are.
We'll go ahead and get the feedback passed along and see if there is anything we can do on our end to improve the information reps reference, and what we can do about the process in general.
Thanks for keeping us updated!