I am really not loving this Samsung support model, which appears to be "let's start a community and let users solve each other's problems and we won't have to do anything at all" and maybe you can hope that Samsung support will see your post and weigh in.
Samsung must develop a way to dismiss persistent notifications. I don't need a constant notification on my watch of a voicemail that does not exist. It wastes battery power and annoys the user. THERE IS NO VOICEMAIL. At the very, very least, the notification should disappear when the voice mailbox is accessed (message or not). There also is absolutely no reason for a notification to be ongoing. If a user saw a notification and chose to ignore it, there is no reason to keep reminding them of it. ONGOING NOTIFICATIONS SHOULD NOT EXIST.
I noticed that even when there is a legitimate voicemail notification, I cannot dismiss it until I access the voicemail. This is wrong. At the very least there should be a setting whereby you can a) select if you want notifications to be ongoing and b) select how many times and at what intervals you want to be reminded, or c) not have an ongoing notification at all.
I understand this particular issue seems to be with the Galaxy Watch 4 and T-Mobile, and T-Mobile says their Samsung tech liaison team is working with Samsung, and Samsung will, supposedly, eventually come up with a fix for it. But it would not be happening in the first place if any of the suggestions above were utilized. I also understand that some people have had success with resolving this issue by factory resetting their watch. I refuse factory reset my watch for a minor issue, especially when the backup data may contain whatever error is causing this, and I don't want to have to go through all my settings again after a reset, just to get my watch to function properly.
Mine was fixed with a specific patch that Samsung only pushes to people with the specific problem. I actually went through the tech support liaison with my carrier (T-Mobile) and one day it was fixed without my being notified that the fix came through. I just assumed it did. In any case here's the link to put in a support ticket with Samsung https://www.samsung.com/us/support/service/ . There are many threads on this here, but someone messaged me the other day and said he (at least for now) solved it this way: "How I fixed it today... Turned off Bluetooth on my phone. Once my watch was disconnected from phone I used my watch to click the notification to check voicemail using my watch. Listened to it telling me I didn't have any voicemail. Hung up. Turned Bluetooth on again. Once my phone connected. No more notifications for voicemails that I don't have. "