i dont think the visual vm apps ever work for unlocked phones. my unlocked s8 and s10 did not, but my vzw note 10 did. i think you can call the carrier and ask them to activate it though.
I've had every Galaxy S phone unlocked since the S7 and have always been able to use Samsungs voicmail app no problem. But I also cannot see it on the S21 Ultra or in the Galaxy store when searching on the S21.
I am having the same problem. my carrier AT&T is telling me Samsung doesn't support that, Samsung is saying my carrier has turned off the feature. I'm about returning the phone. It doesn't seem like an update after all.