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

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.

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 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.

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