If you are a T-Mobile USA subscriber, you may be using "Visual Voicemail" without even realizing it. I discovered that I was using Visual Voicemail when I noticed that voicemail had stopped working for me, on a Google Nexus 4 phone.

To turn off Visual Voicemail, the first step is to have T-Mobile turn it off for you. Then, on your Android phone, do the following (these instructions are for the Marshmallow version of Android):

Go to the phone app, select settings:

Select "Calls":

Select "Voicemail", and turn off "Visual Voicemail":