On iOS toggle the “Silence Unknown Callers” option in the phone settings. You will still get the emblem showing that somebody is actively leaving a voicemail that you can tap on. So you can see what they are saying and can still answer the call while they are leaving the message.

I know that there are Androids that ship natively with stuff like it, and ios stuff too, personally I use KnownCalls, which can just block everyone who isn't a contact.

Pretty sure similar functionality is built-in to iOS. I turned it on for my elderly mother's phone, and it completely stopped the scammers that were constantly calling her. I think it's built-in because I don't remember installing anything.

I use Truecaller on iOS, but silencing unknown numbers is just a setting. I use Truecaller for the AI assistant and screening calls.

https://f-droid.org/en/packages/me.lucky.silence/ is what I use.