I don't think it's possible to block callers from leaving a voicemail from everything I've read.

Maybe @HeavenM might know more.

Hey there @Lunchbox2208! Having our phones close by at all times makes harassing calls even more impactful and blocking the numbers usually helps discourage the caller. They normally get frustrated when they hear the voicemail greeting right away and hang up. Blocking the number on your device settings does not prevent the call from going to voicemail. Since the call does go to the voicemail system, the phone call should show on the call log when you log in to your T-Mobile account online or on the app. If you don’t see it there, then the customer support team would not be able to see the number either. 

If you want to prevent someone from being able to leave a voicemail at all, then you would want to sign up for Family Allowances. 'Never Allowed' numbers hear a 'person you dialed is unable to receive calls at this time' message before the call disconnects.

It is heartbreaking to know that someone is harassing you to that extent, but I really hope this helps you. 

I've had the same issue and this is what worked for me on an android. Tap on the phone icon, click 3 dots at the top, click on settings, scroll down to supplementary services, click on call forwarding, click on forward when busy enter a number to a tablet if you have one on your tmobile acct or any other mobile accts or a landline number that does not have a voicemail set up. The blocked caller will then receive this msg "The service you have attempted has been restricted or is unavailable" I blocked my mom's number and tried it out and it actually worked!!! No more harassing voicemails I hope this works for you and others who are looking for a simple solution!!

When you have blocked someone from calling and it is sent to voicemail, they are then granted the ability to leave a voicemail. In the year of our Lord 2024 if you actually check your voicemail that is a miracle.