Having been a Tmobile employee, they won't be able to tell you how to root your phone and void your warranty. However, I did find a forum and a video on how to proceed. I would recommend backing up everything beforehand, and reading all the instructions carefully first:highonandroid.com/android-smartphones/how-to-root-galaxy-s9-or-s9-plus-exynos-supersu/
If you already have a third party app like Google Voice installed, you can go into your dialer and hit the three dot menu then settings. At the bottom of that page are the settings for Voicemail.
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