Visual voicemail is sadly a tool that carriers use to convince people to buy branded devices. Lots of carriers restrict Visual Voicemail to device that are locked to their carrier, and the sim card will actually disable the app from working if it is installed in an unlocked variant of the same device. You can call in and complain to your carrier, but the person answering the phone will have no idea what you are talking about as they don't even inform their own employees that they do this.
Actually this is still true. It does not happen to all unlocked phones. I am on an unlocked S24 ultra on AT&T and have it,but others don't. Just because it is not true for you does not mean it is not true for others.
I have always used unlocked phones on Verizon network without any problems with visual voicemail. The error pops up if your on wifi. Turn wifi off and use mobile data to set it up.
