Fake Apple calls leaving voicemails. Is there any way to totally block so they can't leave messages?

For quite some time, this topic has popped up online but it always seems to just be a discussion with no real solution. Spam calls trying to mask themselves as Apple support, trying to get you to call back on a number (many times 315 area code but varies). They will leave massively long voicemails at times, too. But here's something new - their blanketing your phone with calls. I'm talking about getting from 15-25 calls PER DAY from varying source numbers, all with the same recording/voicemail.

If you just block each one, within minutes you get another one from a difference source number. Block it, another from a different one, etc etc. They seem to have an unlimited number of call-from numbers this time and it's easy for your voicemail to fill up within a few hours if you don't keep going in to erase them.

So, my question is this: Is there ANY built-in iPhone method (or, third party app) available that completely blocks unknown call numbers from being able to even leave a voicemail? I know you can do "block unknown callers" in-phone but they can still leave voicemails. I'm trying to find a way to intercept unknown numbers before THEY even hear a ring and it just cuts them off, blocks them, etc. Ideas? Thanks.