Visual Voicemail Not Working After iCloud Restore

I recently switched from Google Fi to T-Mobile, and in the process, I upgraded from an iPhone 11 to an 11 Pro. Everything has been smooth, except I can't set up Visual Voicemail in the Phone app after restoring from iCloud backup. Notably, it works if it's a clean install of iOS, but if I restore from my backup, I get the message:

It's bizarre that I can access/setup VVM from a clean install, but not an iCloud backup, as I thought that Phone-specific settings weren't restored. I worked with a T-Mobile tech for several hours today and had him reset my voicemail and network settings from his side, reinitiate the switch for VM, and I've reset my network settings on the phone after the iCloud restore. I even tried Reset All Settings (while keeping my content), but that didn't work either. The only way I can get to it is if I use a clean install of iOS 13.5., and I really don't want to have to set the whole phone up from scratch.

Is there some way to reset the Phone.app settings (I assume it holds the VM settings) to factory settings without doing a complete clean install? That neither the Reset Network Settings nor the Reset All Settings fixed it tells me that something is broken in the Phone.app itself.