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They spoof numbers. It almost always is a number from my state or the area code, very rarely do I get calls from out of state and they are all already labeled "spam risk/telemarketer" when they come in.
Also thank you for writing this comment, I was going to say ios had no way to block unknown numbers, but I searched to double check myself and they finally do have that option! It may have been around for quite awhile, I hadn't checked for a long while.
You can attach an audio file to a post on mastodon and it gets an embedded player. Just federate a compatible audio file?
https://mastodon.art/@Ossi/111138453327480717