It used to be the wild west but FCC is tightening up (maybe even going too far, we'll see). STIR/SHAKEN and KYC (know your customer) rules are making it more expensive for providers to allow any traffic over their networks. Shady providers would look the other way at spammers pumping traffic (providers getting paid); shady providers mix legitimate traffic in so upstream carriers can't just block them, etc.

Now, there's more regulatory teeth to go after the shady providers allowing this traffic.