Why are blocked numbers still able to leave a voicemail?

I thought the whole point of blocking someone was to prevent no contact in the future. Blocked numbers should never be able to call you.
I hear some carriers could have a number blocking feature but that costs extra, and it’s not cheap.
I called my carrier a long time ago regarding blocking numbers and they weren’t very helpful. I could either change my number or disable voicemail completely.

I’ve been looking for an android call blocking app that will allow the phone to disconnect the second a blocked number calls. So far, doesn’t seem to be many apps that do this.