Just my opinion but reading posts on other cellphone forums implementing Visual Voicemail is a hit or miss kind thing. I'd be wary of it.
But getting back to your regular voicemail problem, this is not likely to fix things but try going into your Settings >> Apps menu, find and open the Phone app entry and clear the app's cache.
When you contacted Sprint, outside of that Visual Voicemail suggestion I'm surprised they didn't offer to wipe your voicemail password on their end. That should prompt you to re-enter a new one when you try using voicemail again (enter 1 on the keypad). If you log into your account using Sprint's web site are there any options available to reset your voicemail password from there?

 

Odd that a security update apparently messed up something like voicemail. (A version upgrade maybe, but even that is more of an isolated issue) This probably won't solve your problem either but when a version upgrade that results in some kind of quirky behavior it's often recommended to wipe the system cache partition so you might want to try that. The system cache partition is separate from your data partition so this won't affect your data at all, even if it doesn't help it doesn't hurt to at least try. You need to restart your phone into its Recovery mode to wipe the system cache. This guide walks you through the process:
https://www.hardreset.info/devices/google/google-pixel-3/recovery-mode/