I believe I've seen you post on another thread but are you using a T-Mobile branded device or one you brought from Verizon? If so, have you tested to see if it works in a T-Mobile phone?
I switched from AT&T and lost visual voicemail, then it came back on my iPhone 8 Plus, but not on my wife's 8 or my daughter's 6s. How can I get it on all phones? I'm on the T-Mobile ONE Amplified plan with all three phones on the same account