Several people I know have been having the same issue on IPhones 6S, 8 and 8+. I was told by T-Mobile that it is a IOS 12.1 software issue. Had the same issue last year with IOS 11 that was finally resolved with an update. Looks like they broke it again in IOS 12.1. I hope Apple pushes a update soon.
I had a chance to chat with Marissa to see what the ultimate fix was for the VVM (visual voicemail issues). From talking with her it sounds like this is a provisioning issue between the Voicemail to Text feature and the Visual Voicemail feature on iPhones. The solution was to remove the voicemail to text feature in our backend system to allow the visual voicemail to work. I recommend chatting with T-Force via Facebook or Twitter to have them review your account and features.
I have been dealing with this since I switched my iPhone X (Verizon Model) over to T-Mobile about 5 months ago. Thankfully I have had it fixed twice now and will share how it worked. THE FIRST TIME it happened was right when I switched, and it took about a WEEK to get it resolved, I was about to cancel it all and return to the GOLD DIGGING company I switched from (Verizon). The fix happened by having a rep turn off Voicemail to Text feature on my OnePlus plan. But it seems that depending who you talk to they say they cannot do that without cancelling OnePlus.
This next time happened today, I updated to iOS 12.2 Beta 2 and all hell broke loose and my Visual Voicemail Disappeared again. (just the screen that says "Call Voicemail") After a few hours trying the same thing over finally I asked for a Tech Agent (The first person you talk to will just know basic troubleshooting, but you need to have them move you to an "Expert") He said he ran a "Location Cancellation" on my line, pretty much remotely resetting my phone so it cannot talk to any towers and then reactivating it so it can again. Obviously it hung up the call and then I restarted my phone and when I went to VVM again, it said "Set Up Voicemail" so I entered a new password and recorded a greeting and now it works. I hope this helps some people having issues with a Verizon build iPhone X...
Let me know if anyone has questions!
PS : I want to be clear that before the "Location Cancel" I had reset my phones network settings at least 5 times, power cycled my phone a million times and also activated a new TMobile SIM and none of these worked....Make sure to ask for a Location Cancel!